Dualism (Stanford Encyclopedia of Philosophy)

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Dualism
First published Tue Aug 19, 2003; substantive revision Fri Oct 17, 2025
This entry concerns dualism in the philosophy of mind. The term
‘dualism’ has a variety of uses in the history of ideas.
In general, dualism is the view that, for some particular domain,
there are two fundamental kinds. In theology, for example a
‘dualist’ is someone who believes that Good and Evil
– or God and the Devil – are independent and more or less
equal forces in the world. In the philosophy of mind, dualism is the
theory that mind and body – or the mental and the physical
– are, in some fundamental sense, different kinds of things.
Dualism contrasts with monism, which says that there is only one
fundamental kind; and, rather less commonly, with pluralism, which is
the view that there are many fundamental kinds. Dualism usually enters
philosophy as a response to the mind-body problem, where its main
competitor is materialism, the form of monism that says that mind and
body are both ultimately physical. Dualism is very common in the
history of ideas. Today, dualism is the second most popular response
to the mind-body problem among professional philosophers, after
materialism and ahead of idealism and panpsychism.
1. Dualism and the Mind-Body Problem
2. The History of Dualism
2.1 Dualism in Indian Philosophy
2.2 Dualism in Greek Philosophy
2.3 Dualism in the Abrahamic World
2.4 Dualism and Mechanistic Science
3. Varieties of Dualism
3.1 Substance Dualism
3.2 Property Dualism
3.3 Interactionism
3.4 Epiphenomenalism
3.5 Parallelism
3.6 Hylomorphic Dualism
4. Arguments for Dualism
4.1 The Knowledge Argument
4.2 The Conceivability Argument
4.3 The Argument from Personal Identity
4.4 The Aristotelian Argument in Modern Form
4.5 The Argument from Predicate Dualism
4.6 From Property Dualism to Substance Dualism
5. Problems for Dualism
5.1 The Queerness of the Mental
5.2 The Unity of the Mind
5.2.1 Unity and Bundle Dualism
5.2.2 Unity and Substance Dualism
Bibliography
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Related Entries
1. Dualism and the Mind-Body Problem
Humans have, or seem to have, the sort of properties attributed in the
physical sciences. These physical properties include size, weight,
shape, colour, and motion through space and time. Humans also have, or
seem to have, mental properties, which we do not attribute to typical
physical objects. These mental properties include perceptual
experience, emotional experience, beliefs, desires, agency, and
subjecthood or selfhood. In its broadest form, the mind-body problem
is the problem of explaining how humans and other mentally endowed
beings end up having, or seeming to have, these very different
characteristics (Weir 2023, 1–7). In a more specific form, which
takes centre stage after the emergence of mechanistic science in the
seventeenth century, the mind-body problem is the problem of
reconciling the apparent ontological disparity between mind and body
with the fact that they seem to interact closely (Westphal 2016,
1–3, 43).
The problem has a number of components. These include the ontological
question, what are mind and body, and is one ultimately reducible to,
or in some other sense ‘nothing over and above’ the other?
They also include the causal question: do physical states influence
mental states, do mentals states influence physical states, and if so,
how? Different aspects of the mind-body problem arise for different
aspects of the mental, such as consciousness, intentionality, agency,
personal identity, and subjecthood.
Dualism is one of three classic responses to the mind-body problem
alongside materialism and idealism (Wolff 1751). Materialism says that
the mind, insofar as it truly exists, is fundamentally physical.
Idealism is the converse view that matter, insofar as it truly exists,
is fundamentally mental. Dualism is the view that mind and body are
both real and fundamentally distinct. A fourth option which has gained
prominence in recent discussions is a kind of panpsychism according to
which the things described by physical science are deep-down, partly
or wholly, mental in nature (Freeman 2006, Alter and Nagasawa 2015,
Goff and Moran 2022). There is some disagreement as to whether these
panpsychist theories form a subclass of one or more of the three
classic views, or a distinctive class of their own (Weir 2023,
87–91). For an extensive taxonomy of responses to the mind-body
problem as it concerns consciousness see Kuhn (2024).
Dualism usually enters philosophical discussions as one possible
response to the mind-body problem, especially as it pertains to
consciousness. A 2020 survey suggests that dualism is, in this
context, the second most popular response to the mind-body problem
among professional philosophers (Bourget and Chalmers 2023).
Twenty-two percent of respondents indicated that they ‘accept or
lean towards’ dualism, compared to fifty-one percent for
materialism and eight percent for panpsychism. Idealism was not
included as a possible response to this question (though seven percent
indicated that they accept or lean towards idealism in response to a
question about external-world scepticism).
More recently, dualism has also been discussed in the context of the
‘meta-problem of consciousness’. This is the problem of
explaining why people report dualist intuitions about consciousness,
or why dualism about consciousness seems true, irrespective of whether
it is actually true (Chalmers 2018, Rickabaugh and Moreland 2023,
83–86). The meta-problem of consciousness arises because there
is general agreement that dualism is intuitively appealing, even among
its opponents (see Taliaferro 1994, 26 fn. 10). Analogous
‘meta-problems’ concerning other aspects of the mind-body
problem such as intentionality and personal identity exist but have
not yet received comparable attention. A final area where dualism is
frequently discussed is in the philosophy of religion, where responses
to the mind-body problem intersect with religious views about the
soul.
Other entries which concern aspects of the mind-body problem include
(among many others):
behaviorism
consciousness
eliminative materialism
epiphenomenalism
functionalism
identity theory
intentionality
mental causation
neutral monism
panpsychism
and
physicalism
2 History of Dualism
An early form of dualism can be found in the idea that humans possess
a psychological core, the self or soul, that survive bodily death.
Versions of this idea appear in some of the earliest surviving
sources, including Shang Dynasty oracles, the
Epic of
Gilgamesh
, the Homeric poems, the Hebrew Bible and Egyptian
funerary texts. Religious practices involving communication with the
spirits of the dead appear to be universal across cultures (Steadman
et al. 1996). Data from infant psychology suggest that dualism of this
kind may be part of how humans naturally conceive of themselves,
irrespective of environmental and cultural factors (Wellman 1990, 58;
Bering 2006; Bloom 2007; cf. Hodge 2008). Early philosophical
discussions of dualism emerge in Vedic India and Classical Greece.
Both traditions have a powerful influence on philosophy of mind today.
Works from a third major centre of ancient philosophical texts, China,
also regularly presuppose dualism (Slingerland 2013). However, early
Chinese philosophy is less preoccupied with the mind-body problem than
that of India and Greece. Two events have a profound impact on
subsequent discussions of dualism: the spread of the Abrahamic
religions in late antiquity; and the emergence of mechanistic science
in the early modern period.
2.1 Dualism in Indian Philosophy
A central focus of India’s foundational philosophical texts, the
Upanishads
(c. 700–500 BCE), is the nature and destiny
of the soul (
atman
). The
Upanishads
present the
theory that the soul is an unchanging and imperishable entity that
survives bodily death and undergoes successive reincarnations
(Olivelle 1998). This view would be taken up by numerous subsequent
Indian philosophical traditions. A description of the same idea from
the
Bhagavad Gita
reads:
There has never been a time when you and I… have not existed,
nor will there be a time when we will cease to exist. As the same
person inhabits the body through childhood, youth, and old age, so too
at the time of death he attains another body. The wise are not deluded
by these changes… The body is mortal, but that which dwells in
the body is immortal and immeasurable. (Ch. 2, 12–18)
Two of the six ‘orthodox’ Indian philosophical schools,
Samkhya and Vaisheshika, articulate detailed dualist theories which
are further developed by their sister schools, Yoga and Nyaya. Of the
remaining two ‘orthodox’ schools, Mimamsa appears to
presuppose dualism, while focussing on other matters, and Vedanta
develops both dualist and idealist sub-schools (Radhakrishnan and
Moore 1998).
Of the two surviving ‘heterodox’ Indian philosophical
schools—those that do not recognise the authority of Hindu
scripture—Jainism advances a distinctive form of dualism, while
Buddhism develops sub-schools with various theories of the mind. The
texts of a third ‘heterodox’ school, Carvaka, are mainly
lost, but fragments and second-hand reports show that it embraced a
form of materialism.
Important differences exist between classical Indian forms of
mind-body dualism. For example, Samkhya and Yoga represent the soul
purusha
) as pure awareness. Vaisheshika and Nyaya, by
contrast, attribute to the soul (
atman
) not just
consciousness, but also volition and cognition. All four represent the
individual soul as pervading the cosmos, while Jain texts state that
it (ordinarily) fills only the body that it occupies, like the light
from a lamp filling a room.
A distinctive version of mind-body dualism appears in the early
Buddhist tradition,
Abhidharma
The
Abhidharma Pitaka
(a collection of works by multiple
authors, existing in several recensions) contends that what we mistake
for the soul or self is really a bundle of fleeting atom-events called
‘dharmas’. These dharmas are nonetheless divided into what
appear to be fundamentally mental and fundamentally physical kinds,
making this position a form of dualism, albeit one that departs
radically from other Indian theories in rejecting the idea of an
enduring soul.
The classical Indian dualist theories retain varying degrees of
influence into the present day. Buddhism declined in India and in
Central Asia in the Middle Ages, but flourished in East Asia, carrying
with it a philosophical interest in the mind-body problem and the
nature and (non-)existence of the soul. One result was an early
argument for the fundamentality of consciousness advanced by the Tang
dynasty Buddhist monk, Guifeng Zongmi (780–841 CE):
Moreover, the
qi
of Heaven and Earth is fundamentally lacking
in awareness. If humans are endowed with
qi
that lacks
awareness, how can awareness suddenly arise? Grass and trees are
endowed with the same
qi
, so why do they not have awareness.
On Humanity
, 100)
An important shift in Indian philosophy occurred in the wake of
Shankara’s (c. 8th century CE) exposition of the non-dualist
advaita
) sub-school of Vedanta. Gradually, Advaita Vedanta
gained a prominence unrivalled by other Indian schools (except perhaps
Yoga). This explains the perception in some sources that Indian
philosophical thought leans towards idealism over dualism.
2.2 Dualism in Greek Philosophy
There exists evidence of both mind-body dualism and the cycle of
reincarnation among the Greek pre-Socratics, especially the
Pythagoreans. The resemblance between these ideas and those of Vedic
India has led to speculation about the possibility of a common source
or influence between the two (McEvilly 2002, Ch. 4). Hard evidence for
such a theory remains elusive, however.
The earliest philosophical discussions of dualism in the Greek
tradition to survive in full are those of Plato (c. 429–347
BCE). Plato assumes a distinction between an invisible soul and a
visible body, and argues for both the immortality of the soul, and for
the theory that the soul undergoes successive reincarnations. The
Meno
, the
Phaedo
, and the
Phaedrus
all
argue that humans exhibit knowledge that they could not have obtained
in their current life, presenting this as evidence for the
soul’s pre-existence. The
Phaedo
adds several arguments
that the soul will continue to exist after death. One such argument
appeals to the soul’s affinity with the eternal forms of which,
according to Plato’s theory, material objects are mere copies
(78b4–84b8).
Aristotle (384–322 BCE) develops a very different theory of the
soul. In multiple texts, including the
Physics
Metaphysics
, and
De Anima
, Aristotle analyses
material substances into the matter they are composed of and the form
that organises that matter. For example, Socrates is composed of some
matter, his flesh and bones, organised by the form of a human being.
This theory has come to be known as ‘hylomorphism’ from
the Greek for matter (
hyle
) and form (
morphe
).
Aristotle identifies the soul of a living creature with its form, and
he argues, contra Plato, that forms cannot exist separately from the
material they organise. Aristotle must therefore reject the view that
the soul can survive without the body. However, Aristotle also argues
that one part of the soul, the intellect, is immaterial. He reasons
that a material organ is sensitive only to certain things, just as the
eye is sensitive to light but not sound, whereas the intellect,
according to Aristotle, is sensitive to all forms (
De Anima
429a10–b9). Commentators debate whether this means that, for
Aristotle, the intellect can exist without the body, even if the
entire soul cannot.
Most Western philosophers from antiquity well into the modern era
embrace dualist theories descended from those of Plato and Aristotle.
A third influential Greek tradition for our topic is Epicureanism.
Epicurus (341–270 BCE) following the pre-Socratic Democritus (c.
460–370 BCE) argued that the ‘soul’ is really a
collection of physical atoms that disperse at death (Letter to
Herodotus 10.65). Though its influence waned in late antiquity,
Epicurean materialism underwent a revival in modernity led by figures
ranging from Hobbes to Marx and influenced the development of
contemporary materialism.
Of the other principal Greek schools, the Stoics described the soul as
composed of a special kind of spiritual matter, ‘pneuma’,
which pervades the universe; a view that can be seen as anticipating
contemporary panpsychism. The Sceptics, Academic and Pyrrhonist,
attempted to suspend judgment about such questions.
2.3 Dualism in the Abrahamic World
A factor shaping philosophical developments in the Western half of
Eurasia and in Africa in late antiquity was the expansion of the
Abrahamic faiths. This gave dualism a new intellectual context, and
one that continues to shape philosophical discussions today (e.g.
Warren, Murphey and Maloney 1998; Van Inwagen and Zimmerman 2007;
Farris and Leidenhag 2024). A tradition of harmonising Greek
philosophy with Jewish scripture reached an early apex in the first
century CE in the work of Philo of Alexandria, who drew heavily on
Platonist approaches to the soul. Early Christian thinkers, most
famously St Augustine (354–340), also adopted versions of
Plato’s dualism. Some scholars also credit Augustine with
introducing the conception of the soul as a subjective inner space, an
idea that becomes important in modern discussions (Cary 2000).
Arabic-Islamic philosophy adopted a similar approach beginning with Al
Kindi in the 9th century (Adamson 2018). Thinkers in this tradition
tend to favour a synthesis of (neo-)Platonic and Aristotelian theories
of the soul. One influential contribution was Al Farabi’s (c.
870–950) theory of the active intellect: a cosmic intellect that
stands to the human intellect as the sun stands to the eye. This
theory was further developed by Avicenna (c. 890–1037) and
formed the basis for Averroes’ (1126–1198) doctrine of the
‘unity of the intellect’: the thesis that all humans share
a single intellect. Averroes’s doctrine generated controversy by
casting doubt on the idea of an individual afterlife. Another notable
contribution is Avicenna’s ‘floating man’ thought
experiment, which is sometimes regarded as a precursor to similar
thought experiments in Descartes.
The philosophers of the Latin West inherited this synthesis of
Platonic and Aristotelian dualism. A group at the University of Paris
known as the ‘Latin Averroists’ even adopted
Averroes’ doctrine of the unity of the intellect. The leading
figures of the two main strands of scholastic philosophy, Thomas
Aquinas (1225–1274) and Duns Scotus (c. 1266–1308), both
opposed Averroes’ view, however, with Aquinas dedicating a
polemical treatise to the topic (Adams 1992). At the same time,
Aquinas also objects to the equation of the survival of the soul with
the survival of the self, asserting ‘my soul is not me’
(Commentary on 1 Corinthians, 21:33–4). Commentors disagree
about what this means for Aquinas’s understanding of an
individual afterlife (De Haan and Dahm 2019).
While Aquinas and Scotus agree on the general hylomorphic dualist
framework and in their opposition to Averroes, they disagree on other
issues, especially whether the soul is the only substantial form
organising the matter of the body. Aquinas argues that one substance
can have only one substantial form. Scotus argues that the complexity
of living beings requires multiple substantial forms (Cross 1998,
47–76).
Not all early Abrahamic philosophers embrace straightforwardly dualist
views. The Church father Tertullian (c. 155–220 CE) and the
medieval Jewish philosopher Solomon ibn Gabirol (c.
1021/1022–1058/1070) both argue that the soul is, in a sense,
material (Kitlzer 2013; Pessin 2013). Tertullian and Ibn Gabirol are
at most ambiguous materialists, however. The former is influenced by
the Stoic idea that the soul is composed of a special kind of matter
with fundamentally mental characteristics, while Ibn Gabirol posits a
distinctive kind of ‘incorporeal matter’ to serve as the
material component of spiritual beings. The first philosopher to
combine the worldview of an Abrahamic religion with an unambiguously
materialist response to the mind-body problem may be Hobbes
(1588–1679) in the Early Modern period (Weir 2022,
116–117).
2.4 Dualism and Mechanistic Science
Early Modern philosophy also sees a major development in approaches to
dualism pioneered by Descartes (1596–1650). In addition to
advancing arguments for dualism that remain influential today,
Descartes instigated two shifts in the way philosophers tend to
understand dualism. First, in contrast to his Aristotelian
predecessors, he combined dualism with a new mechanistic conception of
the natural world of which he was a pioneer. Secondly, while earlier
philosophers in the West tend to rest their arguments for the
immateriality of the mind on the intellect, Descartes shifts the focus
to consciousness, which he takes to include not just intellectual
activity but also sensory awareness. It is also in the context of
mechanistic science that the problem of mind-body interaction gains
its status as the leading objection to dualism.
Descartes argues that mind and body are distinct substances
characterised by thought (which for Descartes, includes all conscious
mental states) and extension respectively. By a
‘substance’, Descartes means something that can exist by
itself (Weir 2021a, 281–287). This theory, presented in all of
Descartes’ major works, has served as a standard starting point
for discussions of the mind-body problem ever since.
Descartes’ successors challenge various aspects of his dualist
theory. Elizabeth of Bohemia (1596–1552) questioned whether
Descartes can explain how mind and body interact. Leibniz
(1646–1714) and Berkeley (1685–1753) both attempted to
reduce the material half of Descartes’ ontology to the mental
half, thereby inaugurating a modern tradition of idealist theories.
Hume (1711–1776) substituted a bundle of fleeting perceptions
for the Cartesian mind in a way reminiscent of, and possibly
influenced by, earlier Buddhist theories (Gopnik 2009). Further
details on these debates appear below.
Nonetheless, dualist theories resembling Descartes’ remained
popular. In his 1949 book, The Concept of Mind, Gilbert Ryle was able
to describe Cartesian dualism as the ‘official doctrine’
to which ‘most philosophers, psychologists and religious
teachers subscribe, with minor reservation’ (Ryle 1949, 1). Not
long after the this, however, any such consensus had evaporated. And
although Ryle’s book is often seen as marking the moment when
this shift took place, anti-dualist sentiment had been growing for
some time. Writing in 1930 Arthur Lovejoy states:
The last quarter-century, it may fairly confidently be predicted, will
have for future historians of philosophy a distinctive interest and
instructiveness as the Age of the Great Revolt against Dualism.
(Lovejoy 1930, 1)
If the long-standing predominance of dualism had been under threat for
several decades before Ryle, a stable alternative had not yet been
found. The leading alternatives to dualism in the first half of the
twentieth century were neutral monism, phenomenalism, and
behaviourism, all of which remained minority positions. It is only in
the second half of the twentieth century that contemporary forms of
materialism came to predominate, beginning with U. T. Place (1956) and
J. J. C. Smart (1959).
The ascendance of materialism appears to have been due, in part, to
the mechanistic conception of nature that Descartes had helped
establish. According to the mechanist, the physical world is, as it
would now be expressed, ‘causally closed’. This means that
everything that happens follows from and is in accord with the laws of
physics. Descartes made an exception to accommodate mental causation.
If one does not make such an exception, however, the mechanistic
theory seems to make the mind an epiphenomenon: that is, it is a
by-product of the physical system which has no influence back on it
(see Huxley 1893).
While epiphenomenalism has its supporters, many philosopher have found
it implausible to claim such things as the following; the pain you
feel when you stub your toe, the visual sensations you have when you
see a ferocious lion bearing down on you, or the conscious sense of
understanding you have when you read these words – all have
nothing directly to do with the way you respond. One way to avoid this
conclusion is to adopt the materialist hypothesis that one’s
mental states are one and the same as the physical causes of
one’s behaviours. David Papineau (2001) argues that it is the
emergence of clear-cut evidence for closure under physics, even in the
bodies of living creatures, that accounts for the rise of materialism
in the 1950s and 1960s.
In the period since, the philosophy of mind has been characterised in
large part by attempts to avoid dualism and vindicate materialism.
Even so, some distinguished neuroscientists, such as Charles
Sherrington (1951), Wilder Penfield (1975) and John Eccles (1994)
continued to defend dualism throughout the second half of the
twentieth century. Several influential philosophers also presented
arguments for dualism during this period, most prominently thinkers
like Alvin Plantinga (1970) and Richard Swinburne (1986) who
specialise in philosophy of religion, but also others including Saul
Kripke (1972/1989) and Karl Popper (Popper 1963, 395–408; cf.
Sellars 1954; O’Hear 2025).
Towards the end of the century, a series of challenges to materialism
from a naturalistic perspective led to a revival of property dualism
in mainstream philosophy of mind (e.g. Jackson 1982; Chalmers 1996;
Kim 2005). As a result, a widely accepted opinion in the first decades
of the present century has held that although substance dualism is no
longer a respectable option, property dualism is an important position
that must be taken seriously (see Weir 2023, 31–33). The idea
that property dualism is preferable to substance dualism is itself
regularly challenged, however (Francescotti 2001; Searle 2002;
Zimmerman 2010; Schneider 2012; Lycan 2013; Robinson 2016; Weir
2023).
3. Varieties of Dualism
One natural way to divide up kinds of dualism is in terms of what
sorts of things one chooses to be a dualist about. The most common
categories lighted upon for these purposes are substance and property,
giving one substance dualism and property dualism. A second way is in
terms of the sorts of causal influence between mind and body one
recognises. Interactionism is the view that mind and body act causally
on one another, epiphenomenalism says that that body acts on mind but
mind does not act on body, and parallelism says that mind and body do
not interact. This section reviews these distinctions and says
something about where hylomorphic dualism fits in.
3.1 Substance Dualism
In around the 1970s it became standard practice to draw a distinction
between ‘substance dualism’ and ‘property
dualism’ (Weir 2023, 33). Despite their widespread usage,
however, these terms are rarely defined precisely. It is agreed that
by ‘substance dualism’ we mean the view that minds and
bodies are fundamentally distinct kinds of substances, and that this
is the kind of view exemplified by Plato, Augustine, and Descartes,
and resisted by some more recent dualists such as Jackson (1982) and
Kim (2005). What exactly ‘substance’means in this context
is often left vague, however.
Two traditional senses of ‘substance’ help clarify what is
distinctive of substance dualism. The first defines a substance as an
independent entity: something that can exist by itself (Westphal 2016,
2–3; Weir 2023, 38–44). Substances on this view contrast
with properties, insofar as properties depend on their bearers for
their existence. The second defines a substance as a unitary entity
that persists through time (Benovsky 2009). Substances on this view
contrast with Buddhist or Humean bundles. (To persist through time in
the relevant sense might require that a substance be ‘wholly
present’ at each moment of its existence, in other words, that
it
endures
rather than
perdures
, in the technical
sense of those times. However, the idea of a substance as a persisting
entity predates the the technical distinction between endurance and
perdurance.) Paradigmatic substance dualists such as Plato, Augustine,
and Descartes appear to affirm that the mind is an immaterial
substance in both senses: a unitary, persisting entity, which is
capable of disembodiment and, to that extent, has independent
existence.
The two definitions of substance are logically independent, so someone
might endorse substance dualism in one sense but not the other. For
example, Martine Nida-Rümelin (2012) defends a view which
plausibly qualifies as substance dualism in the sense that it posits a
unitary, immaterial mind, but not in the sense that the mind can exist
independently. Care must therefore be taken to be clear about what one
means when one refers to a position as ‘substance
dualism’. Nida-Rümelin chooses to use the term
‘subject-body dualism’ over ‘substance
dualism’ for this reason.
It is also sometimes assumed that a substance dualist must posit
something like a Lockean ‘substratum’ or a ‘thin
particular’, a component that must be combined with
something’s properties to yield an object. However, it is less
clear that paradigmatic substance dualists posit immaterial substances
in this sense. For example, Descartes argues explicitly that the mind
is a substance in the sense that it exists independently (with the
qualification that nothing can exist without God), and he also
presents the mind as a unitary (indeed indivisible) persisting entity.
But Descartes does not obviously posit anything resembling a Lockean
substratum (Markie 1994, 75–6; Broackes 2006, 160–62;
Madell 2010, 52–56; Weir 2021a, 280–1).
Philosophers sometimes use ‘substance dualism’ to refer to
the kind of dualism that posits an immaterial self as opposed to
immaterial sensations, thoughts, and similar. This is a reasonable
usage insofar as most philosophers take the view that if there are
immaterial selves, these will qualify as substances and that if there
are immaterial mental substances, these will be selves.
However, there is logical space for forms of dualism on which the self
is immaterial, in the sense that it consists in a complex succession
of nonphysical properties, but is not independent, unitary, or
persisting. Likewise, there is logical space for forms of dualism on
which the mind is an immaterial substance that does not, by itself,
qualify as a person or self. Aquinas’s hylomorphic dualism,
described above, may count as an example of the latter. To this
extent, the use of ‘substance dualism’ for forms of
dualism that posit an immaterial self may be misleading.
It is generally accepted that substance dualism is the kind of dualism
that predominates through most of the history of philosophy, and that
may form part of the way humans naturally conceive of themselves
(Bloom 2009). This makes sense, insofar as the traditional idea of a
self or soul that survives bodily death is naturally be taken to
suggest a unitary, persisting and independent entity (Weir 2023,
84–7). On the other hand, Buddhist philosophers have often
combined belief in disembodied survival and reincarnation with the
view of the self as a bundle of fleeting events.
Finally, ‘substance dualism’ is sometimes used
interchangeably with ‘Cartesian dualism’. However, some
substance dualists are keen to distinguish their theories from
Descartes’s. E. J. Lowe (2006) for example, is a substance
dualist, in the following sense. He holds that a normal human being
involves two substances, one a body and the other a person. The latter
is not, however, a purely mental substance that can be defined in
terms of thought or consciousness alone, as Descartes claimed. While
there is widespread agreement that substance dualism of some is
widespread in the history of ideas, it is less clear that this is true
of Cartesian dualism in a narrower sense (Hodge 2008).
The definitions given here do not exhaust those that have been
appealed to in discussions of substance dualism. Lowe himself favours
a distinctive definition in terms not of independence without
qualification, but independence of identity. Swinburne (2019, 13)
defines a substance merely as ‘a component of the world; a
particular object or collection of objects.’
3.2 Property Dualism
Two senses of the term ‘property dualism’ have currency in
contemporary philosophy. The first names a kind of nonreductive
materialism and therefore not a form of dualism in the sense that
contrasts with materialism. The second names a kind of dualism in the
sense that contrasts with materialism, but that aims to avoid the
perceived excesses of substance dualism. We will call the first view
‘predicate dualism’ for the sake of clarity.
Predicate dualism is the theory that mental predicates are (a)
essential for a full description of the world and (b) not reducible to
physicalistic predicates, in a sense of ‘reduction’
defined by Ernst Nagle (1961). For a mental predicate to be reducible
in the sense in question, there must be bridging laws connecting types
of mental states to types of physical states in such a way that all
statements about mental states will be logically entailed by
appropriate statements about physical states.
A standard example is the relationship between water and
O. It is widely accepted that something is water if and
only if it is H
O. It is therefore plausible that every
statement about water is logically entailed by some statement about
O, meaning that any theory concerning water can be
reduced, in Nagel’s (1961) sense, to a theory about
O. (An apparent exception is statements that create
‘intensional’ contexts like ‘Shakespeare knew that
water quenches thirst’. A standard response is that
‘water’ cannot be replaced with
‘H
O’ here only insofar as the sentence really
concerns the mode of presentation under which Shakespeare thinks of
water, rather than water itself.)
Many terms of the special sciences (that is, any science except
physics) are not reducible in this way. Not every hurricane or every
infectious disease, let alone every devaluation of the currency or
every coup d’etat has the same constitutive structure. These
things are defined more by what they do than by their composition or
structure. Such things are multiply realizable; they may be
constituted by different kinds of physical structures under different
circumstances. Because of this, one could not replace these terms by
some more basic physical description and still convey the same
information. There is no particular description, using the language of
physics or chemistry, that would do the work of the word
‘hurricane’, in the way that ‘H
O’
would do the work of ‘water’.
It is widely agreed that many, if not all, psychological states are
similarly irreducible, and so psychological predicates are not
reducible to physical descriptions. This is what is meant by predicate
dualism. The classic source for irreducibility in the special sciences
in general is Fodor (1974), and for irreducibility in the philosophy
of mind, Davidson (1971).
Predicate dualism is perfectly compatible with materialism. Even
though the term ‘pain’, for example, cannot be replaced by
any term from physics or chemistry, it could still be the case that
every particular pain – every pain ‘token’, as it is
expressed – is identical with some physical event in the nervous
system. That is how things are on Davidson’s theory.
The final decades of the twentieth century saw increasing interest in
the stronger position which we call property dualism. Whereas
predicate dualism says that there are two essentially different kinds
of predicates in our language, property dualism says that there are
two essentially different kinds of property out in the world.
Genuine property dualism occurs when, even at the individual level,
the ontology of physics is not sufficient to constitute what is there.
The irreducible language is not just another way of describing what
there is, it requires that there be something more there than was
allowed for in the initial ontology. Until the early part of the
twentieth century, it was common to think that biological phenomena
(‘life’) required property dualism (an irreducible
‘vital force’), but nowadays the special physical sciences
other than psychology are generally thought to involve only predicate
dualism.
In the case of mind, property dualism is usually defended by those who
argue that the qualitative nature of consciousness is not merely
another way of categorising states of the brain or of behaviour, but a
genuinely novel phenomenon. Three classic defences of property dualism
are Jackson (1982), Chalmers (1996) and Kim (2005). These works argue
that consciousness involves properties above everything in the
ontology of the physical science. At the same time, they refrain from
positing immaterial substances.
It should be noted that Chalmers (2010, 139) has clarified that he
intends ‘property dualism’ to entail that there are
nonphysical properties whilst remaining neutral on nonphysical
substances, and he has since expressed a preference for substance
dualism over property dualism (Chalmers 2016, 24), while Jackson
(2003) has abandoned property dualism in favour of materialism.
3.3 Interactionism
Interactionism is the view that mind and body – or mental events
and physical events – causally influence each other. That this
is so is one of our common-sense beliefs, because it appears to be a
feature of everyday experience. The physical world influences my
experience through my senses, and I often react behaviourally to those
experiences. My thinking, too, influences my speech and my actions.
There is, therefore, a natural prejudice in favour of interactionism.
It has been claimed, however, that it faces serious problems.
The simplest objection to interaction is that, in so far as mind and
body are of radically different kinds from each other, they lack that
communality necessary for interaction. It is generally agreed that, in
its most naive form, this objection to interactionism rests on a
‘billiard ball’ picture of causation: if all causation is
by impact, how can the material and the immaterial impact upon each
other? But if causation is either by a more ethereal force or energy
or only a matter of constant conjunction, there would appear to be no
problem in principle with the idea of interaction of mind and
body.
Even if there is no objection in principle, there may be a conflict
between interactionism and some basic principles of physical science.
For example, mind-body interaction seems to require that causal power
flows in and out of the physical body in a way that would conflict
with the conservation of energy, a fundamental scientific law.
Defenders of interactionism have challenged the relevance of the
conservation principle in this context. The conservation principle
states that ‘in a causally isolated system the total amount of
energy will remain constant’. Whereas ‘[t]he
interactionist denies…that the human body is an isolated
system’, so the principle is irrelevant (Larmer 1986, 282; this
article presents a good brief survey of the options). This approach
has been termed conditionality, namely the view that conservation is
conditional on the physical system being closed, that is, that nothing
non-physical is interacting or interfering with it, and, of course,
the interactionist claims that this condition is, trivially, not
met.
That conditionality is the best line for the dualist to take, and that
other approaches do not work, is defended in Pitts (2019) and Cucu and
Pitts (2019). This, they claim, makes the plausibility of
interactionism an empirical matter which only close investigation on
the fine operation of the brain could hope to settle. Papineau (2001)
argues that such evidence was already clear cut in the 1960s. Cucu
(2023), by contrast, claims to find critical neuronal events which do
not have sufficient physical explanation even today. This claim
clearly needs further investigation.
Robins Collins (2011) has claimed that the appeal to conservation by
opponents of interactionism is something of a red herring because
conservation principles are not ubiquitous in physics. He argues that
energy is not conserved in general relativity, in quantum theory, or
in the universe taken as a whole. Why then, should we insist on it in
mind-brain interaction?
Most discussion of interactionism takes place in the context of the
assumption that it is incompatible with the physical world’s
being ‘causally closed’. This is a very natural
assumption, but it is not justified if causal overdetermination of
behaviour is possible. There could then be a complete physical cause
of behaviour, and a mental one. The strongest intuitive objection
against overdetermination is clearly stated by Mills (1996: 112), who
is himself a defender of overdetermination.
For X to be a cause of Y, X must contribute something to Y. The only
way a purely mental event could contribute to a purely physical one
would be to contribute some feature not already determined by a purely
physical event. But if physical closure is true, there is no feature
of the purely physical effect that is not contributed by the purely
physical cause. Hence interactionism violates physical closure after
all.
Mills says that this argument is invalid, because a physical event can
have features not explained by the event which is its sufficient
cause. For example, ‘the rock’s hitting the window is
causally sufficient for the window’s breaking, and the
window’s breaking has the feature of being the third
window-breaking in the house this year; but the facts about prior
window-breakings, rather than the rock’s hitting the window, are
what cause this window-breaking to have this feature.’
The opponent of overdetermination could perhaps reply that the
principle applies, not to every feature of events, but to a subgroup
– say, intrinsic features, not merely relational or comparative
ones. It is this kind of feature that the mental event would have to
cause, but physical closure leaves no room for this. These matters are
still controversial.
Lowe (2008) proposes a different strategy for reconciling mental
causation with the empirical evidence for closure under physics. He
suggests that physical causes always bring about the effects that we
would expect given the laws of physics. But certain physical causes in
the brain do so indirectly, by bringing about an intermediate mental
state that in turn causes the ordinary physical effect. The mental
state and its physical cause are simultaneous, so that there is no
temporal gap in the causal sequence, allowing the mental state to
bring about its physical effect in a way that is invisible to
scientific observation.
The main objection to Lowe’s proposal is that it seems to
require an absurd coincidence, such that the effect of every mental
state just happens to be the effect one would expect its proximate
physical cause to bring about by itself under other circumstances.
Lowe (2008, 77) dismisses this objection, questioning whether it makes
sense to speak of a coincidence at the level of causal laws. An
alternative objection says that Lowe must posit a causal theory that
breaches ordinary standards of theory choice by sacrificing simplicity
for no gain in strength. Ordinarily, if two causal theories have equal
explanatory power, we favour the one that is simpler. This principle
would lead us to favour epiphenomenalism over Lowe’s proposal
(Weir 2021b).
The problem with closure of physics may be radically altered if
physical laws are indeterministic, as quantum theory seems to assert.
If physical laws are deterministic, then any interference from outside
would lead to a breach of those laws. But if they are indeterministic,
interference might produce a result that has a probability greater
than zero, and so be consistent with the laws.
Because it involves assessing the significance and consequences of
quantum theory, this is a difficult matter for the non-physicist to
assess. Some argue that indeterminacy manifests itself only on the
subatomic level, being cancelled out by the time one reaches even very
tiny macroscopic objects: and human behaviour is a macroscopic
phenomenon. Others argue that the structure of the brain is so finely
tuned that minute variations could have macroscopic effects, rather in
the way that, according to ‘chaos theory’, the flapping of
a butterfly’s wings in China might affect the weather in New
York. (For discussion of this, see Eccles (1980; 1987), and Popper and
Eccles (1977).) Still others argue that quantum indeterminacy
manifests itself directly at a high level, when acts of observation
collapse the wave function, suggesting that the mind may play a direct
role in affecting the state of the world (Hodgson 1988; Stapp
1993).
3.4 Epiphenomenalism
Epiphenomenalism is the theory that mental events are caused by
physical events, but have no causal influence on the physical. The
view was given currency by the biologist Thomas Huxley (1893), partly
as a result of his observations of reflex responses in frogs. An
ancient precursor may be found in the Yoga-Samkhya view that the soul
purusha
) consists in pure awareness and has no effect on the
natural world (
prakriti
) (Schweizer 2019).
Epiphenomenalism does not avoid the problem of how two fundamentally
different categories of thing might interact (Green 2003,
149–51). If it is mysterious how the non-physical can have it in
its nature to influence the physical, it ought to be equally
mysterious how the physical can have it in its nature to produce
something non-physical. But epiphenomenalism does promise to avoid the
distinctive problems associated with an immaterial mind acting on a
material body.
There are at least three serious problems for epiphenomenalism. First,
as indicated in section 1, epiphenomenalism is profoundly
counterintuitive. What could be more apparent than that it is the pain
that you feel that makes you cry, or the visual experience of the
boulder rolling towards you that makes you run away? At least one can
say that epiphenomenalism is a fall-back position: it tends to be
adopted because other options are held to be unacceptable.
A second problem, pressed by William James (1890), is that, if mental
states do nothing, there appears to be no reason why they should have
evolved. This objection ties in with the first: the intuition there
was that conscious states clearly modify our behaviour in certain
ways, such as avoiding danger, and it is plain that this would make
them very useful from an evolutionary perspective.
Jackson (1982) replies to this objection by saying that it is the
brain state associated with pain that evolves for this reason: the
sensation is a by-product. Evolution is full of useless or even
harmful by-products. For example, polar bears have evolved thick coats
to keep them warm, even though this has the damaging side effect that
they are heavy to carry.
Jackson’s point is true in general, but does not obviously apply
to the case of mind. The heaviness of the polar bear’s coat
follows directly from those properties and laws which make it warm:
one could not, in any simple way, have one without the other. But with
mental states, dualistically conceived, the situation is the opposite.
The laws of physical nature which, the mechanist says, make brain
states cause behaviour, in no way explain why brain states should give
rise to conscious ones. The laws linking mind and brain are what Feigl
(1958) calls nomological danglers, that is, brute facts added onto the
body of integrated physical law. Why there should have been
by-products of that kind seems to have no obvious evolutionary
explanation.
The third problem concerns the rationality of belief in
epiphenomenalism, via its effect on the problem of other minds. It is
natural to say that I know that I have mental states because I
experience them directly. But how can I justify my belief that others
have them? The simple version of the ‘argument from
analogy’ says that I can extrapolate from my own case. I know
that certain of my mental states are correlated with certain pieces of
behaviour, and so I infer that similar behaviour in others is also
accompanied by similar mental states.
Many hold that this is a weak argument because it is induction from
one instance, namely, my own. The argument is stronger if it is not a
simple induction but an ‘argument to the best
explanation’. I seem to know from my own case that mental events
can be the explanation of behaviour, and I know of no other candidate
explanation for typical human behaviour, so I postulate the same
explanation for the behaviour of others. But if epiphenomenalism is
true, my mental states do not explain my behaviour and there is a
physical explanation for the behaviour of others. It is explanatorily
redundant to postulate such states for others. I know, by
introspection, that I have them, but is it not just as likely that I
alone am subject to this quirk of nature, rather than that everyone
is?
For more detailed treatment and further reading on this topic, see the
entry
epiphenomenalism
3.5 Parallelism
Epiphenomenalists wishes to preserve the integrity of physical science
and the physical world, and appends those mental features that they
cannot reduce. Parallelists preserve both realms intact, but deny all
causal interaction between them. The mental and physical realms run in
harmony with each other, but not because their mutual influence keeps
each other in line. That the two should behave as if they were
interacting would seem to be a bizarre coincidence. This is why
parallelism has tended to be adopted only by those – like
Leibniz – who believe in a pre-established harmony, set in place
by God.
The progression of thought can be seen as follows. Descartes believes
in a more or less natural form of interaction between immaterial mind
and material body. Malebranche thought that this was impossible
naturally, and so required God to intervene specifically on each
occasion on which interaction was required. Leibniz decided that God
might as well set things up so that they always behaved as if they
were interacting, without particular intervention being required.
Outside such a theistic framework, the theory may seem incredible.
Even within such a framework, one might sympathise with
Berkeley’s instinct that once genuine interaction is ruled out
one is best advised to allow that God creates the physical world
directly, within the mental realm itself, as a construct out of
experience. For an argument that even Berkeley’s subjective
idealism falls foul of the problem of mind-body interaction, see Weir
(2021b).
3.6 Hylomorphic Dualism
As noted in section 1.2, hylomorphism is the view that material
substances can be explained in terms of two principles, the matter
they are composed of, and the form that organises that matter:
Socrates is composed of flesh and bones, organised by the form of a
human being. Hylomorphists since Aristotle have identified that soul
of a living creature with its form. This need not entail any form of
mind-body dualism. For a hylomorphist is at liberty to take the view
that a human is a wholly physical composite of matter and form, like a
plant or a rock. However, Aristotle argues that the human soul has an
immaterial part, the intellect, making him to that extent a kind of
dualist. More thoroughgoing forms of dualism can be found among later
hylomorphists such as Thomas Aquinas, who states that the soul is an
immaterial substance (
Summa Theologiae
, Q 75, A2).
It is no surprise, therefore, that some contemporary hylomorphists
describe themselves as ‘hylomorphic dualists’ respecting
the mind-body problem (e.g. Oderberg 2005). There is less agreement
about two further questions: is hylomorphic dualism a kind of
substance dualism, property dualism, or a third theory of neither
kind? And where does hylomorphic dualism stand on the issue of
mind-body interaction?
It might seem clear that Aquinas, at least, should be understood as a
hylomorphic substance dualist. For he himself identifies the soul as
an immaterial substance by name, and describes the soul as a unitary,
persisting entity that is capable of disembodied existence.
At least some scholars are happy to class certain versions of
hylomorphic dualism as hylomorphic substance dualism (Rickabaugh and
Moreland 2023, 326–8). At the same time, there are several
reasons why commentators have resisted the idea that even
Aquinas’s position should be described this way.
First, although Aquinas identifies the soul as a substance, he
describes it as an incomplete substance (Skrezypek 2021). Secondly,
Aquinas does not identify the
self
as an immaterial
substance, and may deny that the self can exist by itself (De Haan and
Dahm 2019). Thirdly, substance dualism is naturally understood as a
view on which both body and soul are substances. However, for Aquinas,
the human body depends on the soul for its existence. For this reason,
if might make greater sense to think of Aquinas as a kind of inverse
property dualist, for whom a human is an immaterial substance with
material properties (Weir 2023, 138–9).
Proponents of hylomorphic dualism contend that it has an advantage
over other kinds of dualism respecting the interaction problem. As we
have seen, some versions of the interaction problem rest on the claim
that it is mysterious how an immaterial mind and a material body could
interact. Hylomorphists can reply that interaction between mind and
body is just one instance of the kind of interaction between form and
matter that goes on throughout nature and is therefore unmysterious
(Feser 2006, 223; 2024, 506–8).
This proposal is not necessarily pertinent to the most influential
version of the interaction problem however, the contention that
empirical evidence shows the material world to be causally closed. An
opponent of hylomorphic dualism might contend that either the soul, on
this view, either has no effect on bodily behaviour or, absurdly,
causes only those behaviours that a mechanist would expect if there
were no such thing as the soul. A natural response for hylomorphic
dualists would be to reject the principle of causal closure itself,
along with the mechanistic, bottom-up picture of the material world
that this principle exemplifies. However, if the empirical case for
closure under physics is less than clear cut this may be of comparable
help to non-hylomorphic dualists, calling into question whether
hylomorphic dualism has a special advantage here.
4. Arguments for Dualism
The most influential arguments for dualism in contemporary philosophy
focus on consciousness, the fact that there is ‘something it is
like’ to be a mentally endowed creature, in Thomas Nagel’s
(1974) famous expression. Of these the most important are the
knowledge argument and the conceivability argument. Two further
motivations for dualism covered here are the argument from personal
identity, and a modern version of the Aristotelian argument. Important
motivations for dualism not covered include arguments from free will
(Goetz 2016), from tense (Pearson 2018), and from the unity of
consciousness (Hasker 1999).
4.1 The Knowledge Argument
One category of arguments for dualism is constituted by the standard
objections against physicalism. Prime examples are those based on the
existence of qualia, one of the most important of which is the
so-called ‘knowledge argument’. Because this argument has
its own entry (see the entry
qualia: the knowledge argument
),
we will deal relatively briefly with it here.
The knowledge argument asks us to imagine a future scientist who has
lacked a certain sensory modality from birth, but who has acquired a
perfect scientific understanding of how this modality operates in
others. This scientist – call him Harpo – may have been
born stone deaf, but become the world’s greatest expert on the
machinery of hearing: he knows everything that there is to know within
the range of the physical and behavioural sciences about hearing.
Suppose that Harpo, thanks to developments in neurosurgery, has an
operation which finally enables him to hear. It is suggested that he
will then learn something he did not know before, which can be
expressed as what it is like to hear, or the qualitative or phenomenal
nature of sound. These qualitative features of experience are
generally referred to as qualia. If Harpo learns something new, he did
not know everything before. He knew all the physical facts before. So
what he learns on coming to hear – the facts about the nature of
experience or the nature of qualia – are non-physical. This
establishes at least a state or property dualism. (See Jackson 1982;
Robinson 1982.)
There are at least two lines of response to this popular but
controversial argument (cf. Goff 2017, 64–74). First is the
‘ability’ response. According to this, Harpo does not
acquire any new factual knowledge, only ‘knowledge how’,
in the form of the ability to respond directly to sounds, which he
could not do before. This essentially behaviouristic account is
exactly what the intuition behind the argument is meant to overthrow.
Putting ourselves in Harpo’s position, it is meant to be obvious
that what he acquires is knowledge of what something is like, not just
how to do something. Such appeals to intuition are always, of course,
open to denial by those who claim not to share the intuition. Some
ability theorists seem to blur the distinction between knowing what
something is like and knowing how to do something, by saying that the
ability Harpo acquires is to imagine or remember the nature of sound.
In this case, what he acquires the ability to do involves the
representation to himself of what the thing is like. But this
conception of representing to oneself, especially in the form of
imagination, seems sufficiently close to producing in oneself
something very like a sensory experience that it only defers the
problem: until one has a physicalist gloss on what constitutes such
representations as those involved in conscious memory and imagination,
no progress has been made.
The other line of response is to argue that, although Harpo’s
new knowledge is factual, it is not knowledge of a new fact. Rather,
it is new way of grasping something that he already knew. He does not
realise this, because the concepts employed to capture experience
(such as ‘looks red’ or ‘sounds C-sharp’) are
similar to demonstratives, and demonstrative concepts lack the kind of
descriptive content that allow one to infer what they express from
other pieces of information that one may already possess. A total
scientific knowledge of the world would not enable you to say which
time was ‘now’ or which place was ‘here’.
Demonstrative concepts pick something out without saying anything
extra about it. Similarly, the scientific knowledge that Harpo
originally possessed did not enable him to anticipate what it would be
like to re-express some parts of that knowledge using the
demonstrative concepts that only experience can give one. The
knowledge, therefore, appears to be genuinely new, whereas only the
mode of conceiving it is novel.
Proponents of the argument respond that it is problematic to maintain
both that the qualitative nature of experience can be genuinely novel,
and that the quality itself be the same as some property already
grasped scientifically: does not the experience’s phenomenal
nature, which the demonstrative concepts capture, constitute a
property in its own right? Another way to put this is to say that
phenomenal concepts are not pure demonstratives, like
‘here’ and ‘now’, or ‘this’ and
‘that’, because they do capture a genuine qualitative
content. Furthermore, experiencing does not seem to consist simply in
exercising a particular kind of concept, demonstrative or not. When
Harpo has his new form of experience, he does not simply exercise a
new concept; he also grasps something new – the phenomenal
quality – with that concept. How decisive these considerations
are, remains controversial.
4.2 The Conceivability Argument
The conceivability argument comes in two forms: the disembodiment
argument, originating in Descartes (
Meditation
V) and the
zombie argument, made popular by Chalmers (1996; 2010). The
disembodiment argument can be put as follows:
It is conceivable that one’s mind might exist without
one’s body.
If it is conceivable that one’s mind might exist without
one’s body then it is possible that one’s mind might exist
without one’s body.
Therefore, it is possible that one’s mind might exist
without one’s body.
Therefore, one’s mind is a different entity from one’s
body.
Descartes’ rationale for accepting premiss (1) can be found in
Meditations I and II where he argues that one can coherently doubt
that anything exists apart from one’s present conscious
existence. Philip Goff summarises Descartes’ line of reasoning
as follows:
The arms and legs you seem to see in front of you, the heart you seem
to feel beating beneath your breast, your body that feels solid and
warm to the touch, all may be figments of a particularly powerful
delusion. You might not even have a brain. … [Nonetheless] you
know for certain that you are a thing that has an experience as of
having arms and legs, a beating heart, a warm, solid body. (Goff 2010,
124)
If one can coherently doubt the existence of anything beyond
one’s conscious mind, then it is apparently conceivable that
one’s mind might exist without one’s body. For Descartes,
premiss (2) rests on the principle that anything one can clearly and
distinctly conceive is possible.
Another important disembodiment argument appears in Kripke (1972,
336–7). Unlike Descartes’ argument, which focusses on the
conceivability that I, as a conscious mind, might exist without a
body, Kripke starts with the claim that it is conceivable that a
particular pain ‘A’ could exist without the particular
brain state with which it is reputed to be identical.
Recent discussion has given greater weight to the second version of
the conceivability argument, defended by Chalmers (1996, 94–9).
Rather than focussing on the idea of a disembodied mind, Chalmers
focuses on the idea of a mindless body. More precisely, he focuses on
the idea of a ‘philosophical zombie’ understood as an
exact duplicate of a conscious human being, without the consciousness.
The zombie argument can be put as follows:
It is conceivable that a physical duplicate of some human should
exist without consciousness (i.e. zombies are conceivable).
If it is conceivable that a physical duplicate of some human
should exist without consciousness, then it is possible that a
physical duplicate of some human should exist without consciousness
(i.e. if zombies are conceivable then zombies are possible).
Therefore, it is possible that a physical duplicate of some human
should exist without consciousness (i.e. zombies are possible).
Therefore, consciousness is something over and above the
physical.
Chalmers (2002) distinguishes two senses in which premiss (1) seems to
be true. First, zombies are conceivable in the sense that we cannot
rule out the possibility of such a scenario a priori: there is no
apparent contradiction in the idea of a physical duplicate of a human
that lacks consciousness. Secondly, zombies are conceivable in the
sense that we seem to be able to positively imagine such a scenario in
sufficient detail to see that it is not contradictory: one can picture
what it would be like for a zombie to exist. For Chalmers, the case
for premiss (2) consists in a theory of the relationship between
semantics and modality to which we return below.
The zombie argument differs from the disembodiment argument because
the hypothesis that the unaltered body could exist without the mind is
not the same as the suggestion that the mind might continue to exist
without the body, nor are they trivially equivalent. The zombie
argument establishes only property dualism and a property dualist
might think disembodied existence inconceivable – for example,
if they thought the identity of a mind through time depended on its
relation to a body (e.g., Penelhum 1970).
There are two main responses to the conceivability argument, in either
form. The first response claims that the scenario described in premiss
(1) is not in fact conceivable. For example, an analytical
behaviourist can argue both that the disembodiment scenario is
conceptually impossible, because the existence of mental states a
priori entails the existence of certain behavioural states, and that
the zombie scenario is conceptually impossible because the existence
of the physical duplicate’s behavioural states a priori entails
the mental states associate with those behaviours. In the context of
the zombie argument, opponents of premiss (1) are known as
‘type-A’ materialists. Chalmers attributes such a view to
Dennett (1991).
Most professional philosophers accept premiss (1) of the zombies
argument (Bourget and Chalmers 2023). Naturally, therefore, most of
the discussion has concerned premiss (2).
The main challenge to premiss (2) says that conceivability is not a
reliable guide to possibility. A rationale of those who think that
conceivability is not a safe indication of possibility is that there
exist a posteriori necessary propositions which we can nonetheless
conceive being false. For example, we seem to be able to conceive a
situation in which Hesperus is not identical to Phosphorus or on which
water is not H
O. And yet if Kripke (1972/1980) is correct,
these are not real possibilities despite their apparent
conceivability. Another way of putting this point is that there are
many epistemic possibilities which are conceivable because they are
epistemic possibilities, but which are not real possibilities.
The standard response to this objection is to provide a theory of what
is going in the kind of a posteriori necessities discussed by Kripke,
and to show that the relationship between mind and body cannot be an a
posteriori necessity of the relevant kind.
For example, Chalmers draws a distinction between two dimensions of
the meaning of a term, the ‘primary intension’ and the
‘secondary intension’. Take the case of
‘water’. There is a sense in which we can imagine a
scenario in which it turns out that water is not H
O but
XYZ. And yet, there is also a sense in which, following Kripke
(1972/1980), we think it is impossible that anything but
O could be water. What is going on here? Chalmer’s
answer is that the term ‘water’ has a primary intension,
which is the function from the actual world to whatever meets our
everyday understanding of water: roughly, the clear, drinkable liquid
in lakes and rivers etc. ‘Water’ also has a secondary
intension which is the function from a world considered as
counterfactual to whatever, in that world, shares the underlying
essence of water in the actual world.
The sense in which it is true that water could be XYZ is that there
are possible scenarios in which the clear drinkable liquid in lakes
and rivers really is XYZ. Were such a scenario actually to obtain, we
would rightly say ‘water is XYZ’ because XZY is what the
primary intension of ‘water’ would pick out. The sense in
which it is impossible that water could be XYZ is that, given the
empirical fact that water is actually H
O, the secondary
intension of ‘water’ picks out H
O in all
counterfactual scenarios.
While there are many details to fill in, the important point is that
when we seem to conceive of water not being H
O, we do not
conceive of an impossible scenario. We conceive of a possible scenario
in which the extension of the term ‘water’ is different.
This can happen because the primary and secondary intensions of
‘water’ differ.
Might we not be doing something similar when we seem to conceive of a
philosophical zombie? Chalmers’ argues that this cannot be so.
For it would require that the terms we use to describe conscious
experience have different primary and secondary intensions, and they
do not.
The primary intension of the term ‘pain’, for example,
picks out a certain feeling – that of hurting – just as
the primary intension of ‘water’ picks out the clear
drinkable liquid in lakes and rivers. But the secondary intension of
‘pain’ does not pick out some underlying essence that
might be present without the primary-intension property of hurting, in
the way that the secondary intension of ‘water’ does pick
out H
O. Rather, the secondary intension of
‘pain’ picks out exactly the same thing as the primary
intension: anything that feels like pain.
Another way of putting this is to say that what we mean by
‘water’ is whatever has the actual underlying essence of
the stuff that appears a certain way. Whereas what we mean by
‘pain’ is just whatever appears (or feels) a certain way:
the primary and secondary intensions of ‘pain’ and other
terms for conscious experiences are the same.
The upshot is that when we seem to conceive of a physical duplicate of
a conscious human without conscious experiences we can be confident
that we are conceiving of a genuinely possible scenario. There is no
possibility that we are failing to recognise that the underlying
essence of consciousness is present in this scenario. For when we talk
about consciousness we are not talking about some underlying essence
that stands to our experiences as H
O does to water but
about the experiences themselves.
Other strategies for defending the inference from conceivability to
possibility have been put forward by Swinburne (1997, New Appendix C;
2018), Goff (2017), and by Kripke (1972/1980) himself, among others.
While the details differ, they share the basic structure, analysing
what is going on in the kind of a posteriori necessity identified by
Kripke, and then showing that the relationship between mind and body
cannot be an instance of that kind of a posteriori necessity.
Two kinds of response exist. The first says that there is some other
reason why we are mistaken when we take ourselves to be able to
conceive of disembodied minds or of ghosts. The most influential
version of this response says that the relevant mental terms are, or
behave like, demonstratives or indexicals, which pick out their
referent without conveying any information about them. This response
is especially powerful regarding Descartes’ original
articulation of the disembodiment argument, which uses the indexical
‘I’. For a disembodiment argument that does not use
demonstratives or indexicals, see Weir (2023, 103–6).
To wield the same response against a conceivability argument that does
not use demonstratives or indexicals it is necessary to argue that
phenomenal concepts behave like demonstratives. We saw this kind of
response to the knowledge argument above. As in that case, the main
reply is that phenomenal concepts seem to capture the qualitative
nature of their referents, in which case they are not relevantly like
demonstratives.
The other option is to argue that while there is no special reason why
the apparent possibilities described in the conceivability argument
may turn out to be impossible, this could still be the case, simply
because there is no strong connection between conceivability and
possibility. This response is sometimes expressed by saying that the
entailment from mind to body or body to mind might be a ‘strong
necessity’ (Chalmers 2002, 36). For an argument that the
existence of strong necessities is itself a priori impossible, see
Cleevely (2022).
4.3 The Argument from Personal Identity
There is a long tradition, dating at least from Reid (1785), of
arguing that the identity of persons over time is not a matter of
convention or degree in the way that the identity of other (complex)
substances is and that this shows that the self is a different kind of
entity from any physical body. Criticism of these arguments and of the
intuitions on which they rest, running from Hume to Parfit (1970:
1984), have left us with an inconclusive clash of intuitions.
A related argument which may have its first statement in Madell
(1981), does not concern identity through time, but the consequences
for identity of certain counterfactuals concerning origin. It can,
perhaps, therefore, break the stalemate which faces the debate over
diachronic identity. The claim is that the broadly conventionalist
ways which are used to deal with problem cases through time for both
persons and material objects, and which can also be employed in cases
of counterfactuals concerning origin for bodies, cannot be used for
similar counterfactuals concerning persons or minds.
Concerning ordinary physical objects, it is easy to imagine
counterfactual cases where questions of identity become problematic.
Take the example of a particular table. We can scale counterfactual
suggestions as follows:
This table might have been made of ice.
This table might have been made of a different sort of wood.
This table might have been made of 95% of the wood it was made of
and 5% of some other wood.
The first suggestion would normally be rejected as clearly false, but
there will come a point along the spectrum illustrated from (i) to
(iii) where the question of whether the hypothesised table would be
the same as the one that actually exists have no obvious answer. It
seems that the question of whether it ‘really’ is the same
one has no clear meaning: it is of, say, 75% the same matter and of
25% different matter; these are the only genuine facts in the case;
the question of numerical identity can be decided in any convenient
fashion, or left unresolved. There will thus be a penumbra of
counterfactual cases where the question of whether two things would be
the same is not a matter of fact.
Let us now apply this thought to conscious subjects. Suppose that a
given human individual had had origins different from those which he
in fact had such that whether that difference affected
who he
was
was not obvious to intuition. What would count as such a case
might be a matter of controversy, but there must be one. Perhaps it is
unclear whether, if there had been a counterpart to Jones’ body
from the same egg but a different though genetically identical sperm
from the same father, the person there embodied would have been Jones.
Some philosophers might regard it as obvious that sameness of sperm is
essential to the identity of a human body and to personal identity. In
that case imagine a counterpart sperm in which some of the molecules
in the sperm are different; would that be the same sperm? If one
pursues the matter far enough there will be indeterminacy which will
infect that of the resulting body. There must therefore be some
difference such that neither natural language nor intuition tells us
whether the difference alters the identity of the human body; a point,
that is, where the question of whether we have the same body is not a
matter of fact.
How one is to describe these cases is, in some respects, a matter of
controversy. Some philosophers think one can talk of
vague
identity
or
partial identity
. Others think that such
expressions are nonsensical. There is no space to discuss this issue
here. It is enough to assume, however, that questions of how one is
allowed to use the concept of identity effect only the care with which
one should characterize these cases, not any substantive matter of
fact. There are cases of substantial overlap of constitution in which
that
fact is the only bedrock fact in the case: there is no
further fact about whether they are ‘really’ the same
object. If there were, then there would have to be a
haecceitas
or
thisness
belonging to and
individuating each complex physical object, and this we are assuming
to be implausible if not unintelligible. (More about the conditions
under which
haecceitas
can make sense will be found
below.)
One might plausibly claim that no similar overlap of constitution can
be applied to the counterfactual identity of minds. In Geoffrey
Madell’s (1981) words:
But while my present body can thus have its partial counterpart in
some possible world, my present consciousness cannot. Any present
state of consciousness that I can imagine either is or is not mine.
There is no question of degree here. (91)
Why is this so? Imagine the case where we are not sure whether it
would have been Jones’ body – and, hence, Jones –
that would have been created by the slightly modified sperm and the
same egg. Can we say, as we would for an object with no consciousness,
that the story
something the same
something
different
is the whole story: that overlap of constitution is all
there is to it? For the Jones body as such, this approach would do as
well as for any other physical object. But suppose Jones, in
reflective mood, asks himself ‘if that had happened, would I
have existed?’ There are at least three answers he might give to
himself. (i) I either would or would not, but I cannot tell. (ii)
There is no fact of the matter whether I would or would not have
existed: it is just a mis-posed question. (iii) In some ways, or to
some degree, I would have, and in some ways, or to some degree, I
would not. The creature who would have existed would have had a kind
of overlap of psychic constitution with me.
The third answer parallels the response we would give in the case of
bodies. But as an account of the subjective situation, it is arguable
that this makes no sense. Call the creature that would have emerged
from the slightly modified sperm, ‘Jones2’. Is the overlap
suggestion that, just as, say 85% of Jones2’s original body
would have been identical with Jones’, about 85% of his psychic
life would have been Jones’? That it would have been
like
Jones’ – indeed that Jones2 might have had a
psychic life 100% like Jones’ – makes perfect sense, but
that he might have been to that degree, the same psyche – that
Jones ‘85% existed’ – arguably makes no sense. Take
the case in which Jones and Jones2 have
exactly
similar lives
throughout: which 85% of the 100% similar mental events do they share?
Nor does it make sense to suggest that Jones might have participated
in the whole of Jones2’s psychic life, but in a rather ghostly
only
85% there manner. Clearly, the notion of overlap of
numerically identical psychic parts cannot be applied in the way that
overlap of actual bodily part constitution quite unproblematically
can.
This might make one try the second answer. We can apply the
‘overlap’ answer to the Jones body, but the question of
whether the minds or subjects would have been the same, has no clear
sense. It is difficult to see why it does not. Suppose Jones found out
that he had originally been one of twins, in the sense that the zygote
from which he developed had divided, but that the other half had died
soon afterwards. He can entertain the thought that if it had been his
half that had died, he would never have existed as a conscious being,
though someone would whose life, both inner and outer, might have been
very similar to his. He might feel rather guiltily grateful that it
was the other half that died. It would be strange to think that Jones
is wrong to think that there is a matter of fact about this. And how
is one to ‘manage’ the transition from the case where
there is a matter of fact to the case where there is not?
If the reasoning above is correct, one is left with only the first
option. If so, there has to be an absolute matter of fact from the
subjective point of view. But the physical examples we have considered
show that when something is essentially complex, this cannot be the
case. When there is constitution, degree and overlap of constitution
are inevitably possible. So the mind must be simple. Given that the
body is high complex, it seems to follow that the mind is
immaterial.
4.4 The Aristotelian Argument in Modern Form
Putting his anti-materialist argument outlined above, in section 1, in
very general terms, Aristotle’s worry was that a material organ
could not have the range and flexibility that are required for human
thought. His worries concerned the cramping effect that matter would
have on the range of
objects
that intellect could
accommodate. Parallel modern concerns centre on the restriction that
matter would impose on the range of rational
processes
that
we could exhibit. Some of these concerns are of a technical kind.
Gödel, for example, believed that his famous theorem showed that
there are demonstrably rational forms of mathematical thought of which
humans are capable which could not be exhibited by a mechanical or
formal system of a sort that a physical mind would have to be. Penrose
(1990) has argued that Turing’s halting problem has similar
consequences. But there are other less technical and easier to
appreciate issues. We will mention four ways in which physicalist
theories of thought seem vulnerable to attack by the dualist.
(a) At least since the time of Ryle’s (1949)
Concept of
Mind
, it has been assumed that thinking can be handled in a
dispositionalist way; so only sensations or ‘raw feels’
constitute a problem for the physicalist. There has been a rise or
revival of a belief in what is now called
cognitive
phenomenology
, that is, the belief that thoughts, of whatever
kind – beliefs, desires, and the whole range of propositional
attitude state – are conscious in a more than behavioural
functional sense. This raises problems for physicalism, for, just as
it is a problem that direct knowledge of ‘what it is like’
to experience your sensations is ultimately hidden from anyone else,
so
what you are thinking
is directly accessible only
privately, once it has been conceded that it has a phenomenology and
not just a functional manifestation.
(b) Anything purely physical operates solely according to physical
laws operating on its physical properties: it does not, at bottom,
operate according to meanings, senses, or propositional content. As a
‘thinking machine’, it is what Dennett (1987, 61) calls
‘a syntactic engine’, where ‘syntax’ is used,
somewhat extendedly, to signify merely the physical structure of
symbols and the consequent rules of their operation, rather than their
meaning. The issue is whether, under this constraint, one can give an
account for meaningful communication and understanding at all.
(c) Physicalist theories of thought almost all focus on the model of
computing, and when it comes to making problem solving (as opposed to
simply calculating) computers, computation comes across what is known
as the Frame Problem. This is clearly expounded in Dennett (1984); see
also the entry on
The Frame Problem
The problem is that the ‘mechanical mind’ can only follow
instructions, cannot see relevance that has not been strictly encoded.
This is often expressed by saying it lacks ‘common
sense’.
(d) There is what has become known as ‘Benacerraf’s
Problem’ (See his 1983; cf. Robinson 2011). Numbers, it would
seem, are abstract objects, yet our intellects operate with them all
the time. How does a physical brain interact with an abstract entity?
A similar problem could be raised for concepts in general; they are
abstract, general entities, not physical particulars, yet they are the
meat and drink of thinking. For a dualist about intellect there does
not appear to be the same problem. The immaterial intellect is
precisely the sort of thing that can grasp abstract objects, such as
numbers and universals – in the Aristotelian context, the
immaterial intellect is the home of forms. (There is still the issue
of how this intellectual capacity of the immaterial mind relates to
sensory consciousness. According to Aristotle, perception is a wholly
embodied process, but for modern dualists, sensory consciousness is
not material. In order to unify the perceptual and intellectual
functions of the mind, traditional empiricists tended to be imagists,
in their theory of thought, in order to assimilate the intellectual to
the sensory, but this assimilation is rejected by those who believe in
a distinct cognitive phenomenology, as mentioned in (a) above.) The
difficulty in accounting for the brain’s relation to abstract
entities explains why most materialists tend to be nominalists, thus
reducing thoughts to concrete particulars of some kind. (D. M.
Armstrong in his (1978) is a striking exception to this, accepting an
in re
theory of universals.) But if you do not find either
nominalism or Armstrong’s causal-functional theory of thinking
convincing, Plato’s idea in the
Phaedo
that the
intellect must be such that it can have an affinity with immaterial
things may begin to look plausible.
We will not discuss (a) further, as it is discussed in section 5 of
the entry on
phenomenal intentionality
An immaterialist response to (d) can be found in Robinson (2011).
Both (b) and (c) seem to draw out the claim that a material system
lacks understanding. John Searle’s famous ‘Chinese
Room’ argument (Searle 1980; see also the entry on the
Chinese Room Argument
seems to support this conclusion, at least if the material system
takes the form of a classical computer, manipulating symbols according
to rules. Searle imagines himself in a room with a letter box through
which strings of symbols are posted in, and, following a book of
rules, he puts out symbols which the rules dictate, given the strings
he is receiving. In fact, Searle says, he has been conducting a
conversation in Chinese, because the symbols are Chinese script, and
the rules those on which a Chinese computer might work, but he has not
understood a word. Therefore, neither does a computer understand, so
we, understanding creatures, are not computers. If Searle is right,
this is the end for the classical ‘syntactic engine’ as a
model for thought.
Another argument for the same conclusion is advanced by James Ross
(2008, Ch. 6) and Edward Feser (2013). This argument contends that for
the kind of reasons given by Kripke (1982), nothing material can have
an exact or unambiguous conceptual content. For example, nothing about
a machine can ever make it determinately true that it is engaging in
addition, rather than a distinct function, ‘quaddition’
which yields the same results when x and y are smaller than some
arbitrarily high number n, but that yield 5 otherwise. Since the
machine’s internal operations can be interpreted in various
ways, nothing about its physical configuration or computational
history uniquely fixes which rule it is following. If it malfunctions
and starts yielding quaddition outputs rather than addition outputs,
the only thing that makes this count as a malfunction is extrinsic
factors such as its designers’ intentions or our
interpretations. By contrast, human thought processes can have an
exact or unambiguous conceptual content: it is really true that humans
can add rather than quuad. If so, it follows that human thought
processes are not material.
A blow was struck against the computational theory of thought when, in
2000, Fodor produced his
The Mind Does Not Work That Way
, in
which he made clear his belief that the kind of computationalism that
he had been describing and developing ever since the 1970s only fits
sub-personal informational processing, not conscious, problem solving
thought. Fodor had, in fact, always mentioned this reservation, but
his claim that what he was describing was ‘the Language of
Thought’ led his readers generally to take him to be proposing
an account of what we normally consider to be thinking, which is not
restricted to (even if it includes at all) sub-personal processes. The
modest view is entirely in line with his close colleague
Chomsky’s claim that his – Chomsky’s –
linguistic theories cannot touch the ‘creative use of
language’(See Baker, 2011).
A physicalist response to at least some of these challenges is to say
that they apply only to the classical computing model, and are avoided
by connectionist theories. Classical computing works on rules of
inference like those of standard logic, but connectionism is rather a
form of associationism, which is supposedly closer to the way in which
the brain works. (See the entry on Connectionism.) The latest version
of this – ‘Deep Learning’ – has proven
unexpectedly powerful and underlies all the major advances of AI in
recent years, from image generation to chatbots. But Gary Marcus (2018
– see Other Internet Resources) and others have pointed out the
ways in which these impressive machines are quite different from human
thought. Marcus points to ten significant differences, but the two
most easy to capture briefly are (i) ‘deep learning’ is
data hungry. We can learn things with very few trials because we latch
on to abstract relationships, whereas the machine requires many
– perhaps thousands or millions – of examples to try to
catch extensionally what we get by the abstract or intensional
relation. (ii) Deep learning is shallow and has a limited capacity for
transfer of what it has learnt from one context to another, even
though the differences look trivial to us. It may follow that
collecting examples cannot itself constitute ‘getting the
point’, though it might manage to mimic it, if the circumstances
are carefully managed.
The dualist might sum up the situation on thought in the following
way. The case against physicalist theories of sensation is that it is
unbelievable that
what it feels like
to be struck hard on the
nose is itself either just a case of being disposed or caused to
engage in certain behaviours, or that
what it feels like
is
not fundamental to the way you do react. Similarly, the dualist about
thought will say, when you are, for example, engaged in a
philosophical discussion, and you make a response to your
interlocutor, it is obvious that you are intending to respond to
what you thought he or she meant
and are concentrating on
what
what you intend to say
means. It seems as bizarre to say
that this is a byproduct of processes to which meaning is irrelevant,
as it is to claim the same about sensory consciousness. You are, in
other words, as fundamentally a semantically driven engine, as you are
a sensorily consciously driven one.
A dualist might, on this basis, argue that Plato was right in claiming
that intellect necessarily has an affinity with the realm of abstract
entities, and Aristotle was right to think that no material or
mechanical system could capture the flexibility built into genuine
understanding.
4.5 The Argument from Predicate Dualism
We said above that predicate dualism might seem to have no ontological
consequences, because it is concerned only with the different way
things can be described within the contexts of the different sciences,
not with any real difference in the things themselves. This, however,
can be disputed.
The argument from predicate to property dualism moves in two steps,
both controversial. The first claims that the irreducible special
sciences, which are the sources of irreducible predicates, are not
wholly objective in the way that physics is, but depend for their
subject matter upon interest-relative perspectives on the world. This
means that they, and the predicates special to them, depend on the
existence of minds and mental states, for only minds have
interest-relative perspectives. The second claim is that psychology
– the science of the mental – is itself an irreducible
special science, and so it, too, presupposes the existence of the
mental. Mental predicates therefore presuppose the mentality that
creates them: mentality cannot consist simply in the applicability of
the predicates themselves.
First, let us consider the claim that the special sciences are not
fully objective, but are interest-relative.
No-one would deny, of course, that the very same subject matter or
‘hunk of reality’ can be described in irreducibly
different ways and it still be just that subject matter or piece of
reality. A mass of matter could be characterized as a hurricane, or as
a collection of chemical elements, or as mass of sub-atomic particles,
and there be only the one mass of matter. But such different
explanatory frameworks seem to presuppose different perspectives on
that subject matter.
This is where basic physics, and perhaps those sciences reducible to
basic physics, differ from irreducible special sciences. On a realist
construal, the completed physics cuts physical reality up at its
ultimate joints: any special science which is nomically strictly
reducible to physics also, in virtue of this reduction, it could be
argued, cuts reality at its joints, but not at its minutest ones. If
scientific realism is true, a completed physics will tell one
how
the world is
, independently of any special interest or concern:
it is just how the world is. It would seem that, by contrast, a
science which is not nomically reducible to physics does not take its
legitimation from the underlying reality in this direct way. Rather,
such a science is formed from the collaboration between, on the one
hand, objective similarities in the world and, on the other,
perspectives and interests of those who devise the science. The
concept of hurricane is brought to bear from the perspective of
creatures concerned about the weather. Creatures totally indifferent
to the weather would have no reason to take the real patterns of
phenomena that hurricanes share as constituting a single kind of
thing. With the irreducible special sciences, there is an issue of
salience , which involves a subjective component: a
selection
of phenomena with a certain teleology in mind is required before their
structures or patterns are reified. The entities of metereology or
biology are, in this respect, rather like Gestalt phenomena.
Even accepting this, why might it be thought that the perspectivality
of the special sciences leads to a genuine property dualism in the
philosophy of mind? It might seem to do so for the following reason.
Having a perspective on the world, perceptual or intellectual, is a
psychological state. So the irreducible special sciences presuppose
the existence of mind. If one is to avoid an ontological dualism, the
mind that has this perspective must be part of the physical reality on
which it has its perspective. But psychology, it seems to be almost
universally agreed, is one of those special sciences that is not
reducible to physics, so if its subject matter is to be physical, it
itself presupposes a perspective and, hence, the existence of a mind
to
see matter
as psychological. If this mind is physical and
irreducible, it presupposes mind to see it as such. We seem to be in a
vicious circle or regress.
We can now understand the motivation for full-blown reduction. A true
basic physics represents the world as it is in itself, and if the
special sciences were reducible, then the existence of their
ontologies would make sense as expressions of the physical, not just
as ways of seeing or interpreting it. They could be understood
‘from the bottom up’, not from top down. The
irreducibility of the special sciences creates no problem for the
dualist, who sees the explanatory endeavor of the physical sciences as
something carried on from a perspective conceptually outside of the
physical world. Nor need this worry a physicalist,
if
he can
reduce psychology, for then he could understand ‘from the bottom
up’ the acts (with their internal, intentional contents) which
created the irreducible ontologies of the other sciences. But
psychology is one of the least likely of sciences to be reduced. If
psychology cannot be reduced, this line of reasoning leads to real
emergence for mental acts and hence to a real dualism for the
properties those acts instantiate (Robinson 2003).
4.6 From Property Dualism to Substance Dualism
As we noted in section 2.4, there exists a received opinion in recent
philosophy of mind that property dualism is greatly preferable to
substance dualism. Here, we consider one kind of argument that
substance dualism is in fact more defensible. The argument discussed
below presses the case that the mind is a substance in the sense that
it is a unitary entity, not a Buddhist or Humean bundle. For an
argument that dualists must accept that the mind is a substance in the
sense that it is an independent entity, capable of existing by itself,
see Weir (2021c; 2023).
Hume is generally understood to advance what is known as the
‘bundle’ theory of the self (
Treatise
Book I,
Part IV, section VI), according to which there are mental states, but
no further subject or substance which possesses them. He famously
expresses his theory as follows.
…when I enter most intimately into what I call
myself
I always stumble on some particular perception or other, of heat or
cold, light or shade,love or hatred, pain or pleasure. I can never
catch
myself
at any time without a perception, and can never
observe any thing but the perception.
Nevertheless, in the Appendix of the same work he expressed
dissatisfaction with this account. Somewhat surprisingly, it is not
very clear just what his worry was, but it is expressed as
follows:
In short there are two principles, which I cannot render consistent;
nor is it in my power to renounce either of them, viz. that all our
distinct perceptions are distinct existences, and
that the mind
never perceives any real connection between distinct
existences
Berkeley had entertained a similar theory to the one found in
Hume’s main text in his
Philosophical Commentaries
(Notebook A, paras 577–81), but later rejected it for the claim
that we could have a
notion
, though not an
idea
of
the
self
. Something resembling this Berkeleian view is
anticipated by Nyaya responses to the Buddhist bundle theory of the
mind in antiquity. It is expressed in more modern terms by John
Foster.
A natural response to Hume would be to say that, even if we cannot
detect ourselves apart from our perceptions (our conscious
experiences)we can at least detect ourselves in them … Surely I
am aware of [my experience], so to speak, from the inside – not
as something presented, but as something which I have or as the
experiential state which I am in … and this is equivalent to
saying that I detect it by being aware of myself being visually aware.
(Foster 1991, 215)
There is a clash of intuitions here between which it is difficult to
arbitrate. There is an argument that is meant to favour the need for a
subject, as claimed by Berkeley and Foster.
If the bundle theory were true, then it should be possible to
identify mental events independently of, or prior to, identifying the
person or mind to which they belong.
It is not possible to identify mental events in this way.
Therefore the bundle theory is false.
E. J. Lowe (1996) defends this argument and argues for (2) as
follows.
What is wrong with the [bundle] theory is that … it
presupposes, untenably, that an account of the identity conditions of
psychological modes can be provided which need not rely on reference
to persons. But it emerges that the identity of any psychological mode
turns on the identity of the person that possesses it. What this
implies is that psychological modes are essentially modes of persons,
and correspondingly that persons can be conceived of as
substances.
To say that, according to the bundle theory, the identity conditions
of individual mental states must be independent of the identity of the
person who possesses them, is to say that their identity is
independent of the bundle to which they belong. Hume certainly thought
something like this, for he thought that an impression might
‘float free’ from the mind to which it belonged, but it is
not obvious that a bundle theorist is forced to adopt this position.
Perhaps the identity of a mental event is bound up with the complex to
which it belongs. That this is impossible certainly needs further
argument.
Hume seems, however, in the main text to unconsciously make a
concession to the opposing view, namely the view that there must be
something more than the items in the bundle to make up a mind. He
says:
The mind is a kind of theatre where several perceptions successively
make their appearance; pass, re-pass, glide away and mingle in an
infinite variety of postures and situations.
Talk of the mind as a theatre is, of course, normally associated with
the Cartesian picture, and the invocation of any necessary medium,
arena or even a field hypostasize some kind of entity which binds the
different contents together and without which they would not be a
single mind. Modern Humeans – such as Parfit (1971; 1984) or
Dainton (2008) – replace the theatre with a co-consciousness
relation. So the bundle theorist is perhaps not as restricted as Hume
thought. The bundle consists of the objects of awareness
and the
co-consciousness relation (or relations) that hold between them
The modern bundle theorist might say that it is the nexus of
co-consciousness relations that constitutes our sense of the subject
and of the act of awareness of the object. This involves abandoning
the second of Hume’s principles,
that the mind can never
perceive any connection between distinct existences
, because the
co-consciousness relation is something of which we are aware.
5. Problems for Dualism
By far the most influential objection to dualism is the problem of
interaction, which we have already discussed in section 3.3. In this
section we consider two other facets of dualism that worry critics.
First, there is what one might term the
queerness
of the
mental if conceived of as non-physical. Second there is the difficulty
of giving an account of the unity of the mind. Important challenges
not covered here include providing a dualist account of embodiment and
explaining when immaterial minds arise in the gestation of individual
organisms and in the evolution of species.
5.1 The Queerness of the Mental
Physical objects and their properties are sometimes observable and
sometimes not, but any physical object is equally accessible, in
principle, to anyone. From the right location, we could all see the
tree in the quad, and, though none of us can observe an electron
directly, everyone is equally capable of detecting it in the same ways
using instruments. But the possessor of mental states seems to have a
privileged access to them that no-one else can share. That is why
there is a widely recognised sceptical ‘problem of other
minds’, but no comparably well established ‘problem of my
own mind’. This suggests to some philosophers that minds are not
ordinary occupants of physical space.
Physical objects are spatio-temporal, and bear spatio-temporal and
causal relations to each other. Mental states seem to have causal
powers, but they also possess the mysterious property of
intentionality – being
about
other things –
including things like Zeus and the square root of minus one, which do
not exist. No mere physical thing could be said to be, in a literal
sense, ‘about’ something else. The nature of the mental is
both queer and elusive. In Ryle’s deliberately abusive phrase,
the mind, as the dualist conceives of it, is a ‘ghost in a
machine’. Ghosts are mysterious and unintelligible: machines are
composed of identifiable parts and work on intelligible principles.
Considerations of this kind motivate many philosophers to avoid
dualism if possible.
Arguably, this contrast holds only if we stick to a Newtonian and
common-sense view of the material, however. Think instead of energy
and force-fields in a space-time that possesses none of the properties
that our senses seem to reveal: on this conception, we seem to be able
to attribute to matter nothing beyond an abstruse mathematical
structure. Whilst the material world, because of its
mathematicalisation, forms a tighter abstract system than mind, the
sensible properties that figure as the objects of mental states may
constitute the only intelligible content for any concrete picture of
the world that we can devise. So, perhaps the world within the
experiencing mind is, once one considers it properly, no more –
or even less – queer than the world outside it.
5.2 The Unity of the Mind
Whether one believes that the mind is a substance or just a bundle of
properties, the same challenge arises, which is to explain the nature
of the unity of the immaterial mind. For the Cartesian, that means
explaining how we should understand the notion of immaterial
substance. For the Humean, the issue is to explain the nature of the
relationship between the different elements in the bundle that binds
them into one thing. Neither tradition has been notably successful in
this latter task: indeed, Hume, in the appendix to the
Treatise
, declared himself wholly mystified by the problem,
rejecting his own initial solution (though quite why is not clear from
the text).
5.2.1 Unity and Bundle Dualism
If the mind is only a bundle of properties, without a mental substance
to unite them, then an account is needed of what constitutes its
unity. The only route appears to be to postulate a primitive relation
of co-consciousness in which the various elements stand to each
other.
There are two strategies which can be used to attack the bundle
theory. One is to claim that our intuitions favour belief in a subject
and that the arguments presented in favour of the bundle alternative
are unsuccessful, so the intuition stands. The other is to try to
refute the theory itself. Foster (1991, 212–9) takes the former
path. This is not effective against someone who thinks that
metaphysical economy gives a prima facie priority to bundle theories,
on account of their avoiding mysterious substances.
The core objection to bundle theories (e.g. Armstrong 1968,
21–3) is that, because it takes individual mental contents as
its elements, such contents should be able to exist alone, as could
the individual bricks from a house. Hume accepted this consequence,
but most philosophers regard it as absurd. There could not be a mind
that consisted of a lone pain or red after-image, especially not of
one that had detached itself from the mind to which it had previously
belonged. Therefore it makes more sense to think of mental contents as
modes of a subject
Bundle theorists tend to take phenomenal contents as the primary
elements in their bundle. Thus the problem is how to relate, say, the
visual field to the auditory field, producing a ‘unity of
apperception’, that is, a total experience that seems to be
presented to a single subject. Seeing the problem in this way has
obvious Humean roots. This atomistic conception of the problem becomes
less natural if one tries to accommodate other kinds of mental
activity and contents. How are acts of conceptualising, attending to
or willing with respect to, such perceptual contents to be conceived?
These kinds of mental acts seem to be less naturally treated as atomic
elements in a bundle, bound by a passive unity of apperception.
William James (1890, vol. 1, 336–41) attempts to answer these
problems. He claims to introspect in himself a ‘pulse of
thought’ for each present moment, which he calls ‘the
Thought’ and which is the ‘vehicle of the judgement of
identity’ and the ‘vehicle of choice as well as of
cognition’. These ‘pulses’ are united over time
because each ‘appropriates’ the past Thoughts and
‘makes us say “as sure as I exist, those past facts were
part of myself”. James attributes to these Thoughts acts of
judging, attending, willing etc, and this may seem incoherent in the
absence of a genuine subject. But there is also a tendency to treat
many if not all aspects of agency as mere awareness of bodily actions
or tendencies, which moves one back towards a more normal Humean
position. Whether James’ position really improves on
Hume’s, or merely mystifies it, is still a moot point. (But see
Sprigge (1993), 84–97, for an excellent, sympathetic
discussion.)
5.2.2 Unity and Substance Dualism
The problem is to explain what kind of a thing an immaterial substance
is, such that its presence explains the unity of the mind. The answers
given can be divided into three kinds.
(a) The ‘ectoplasm’ account: The view that immaterial
substance is a kind of immaterial stuff. There are two problems with
this approach. First, in so far as this ‘ectoplasm’ has
any characterisation as a ‘stuff’ – that is, a
structure of its own over and above the explicitly mental properties
that it sustains – it leaves it as much a mystery why
this
kind of stuff should support unified conscious
experience as it is why ordinary matter should. Second, and
connectedly, it is not clear in what sense such stuff is immaterial,
except in the sense that it cannot be integrated into the normal
scientific account of the physical world. Why is it not just an
aberrant kind of physical stuff? One answer to the latter question
would be that the immaterial stuff is fundamentally mental and
therefore not physical in the sense relevant to physicalism (Wilson
2006, 92, fn 1).
b) The ‘consciousness’ account: The view that
consciousness is the substance. Account (a) allowed the immaterial
substance to have a nature over and above the kinds of state we would
regard as mental. The consciousness account does not. This is
Descartes’ view. The most obvious objection to this theory is
that it does not allow the subject to exist when unconscious. This
forces one to take one of four possible theories. One could claim (i)
that we are conscious when we do not seem to be (which was
Descartes’ view): or (ii) that we exist intermittently, though
are still the same thing (which is Swinburne’s theory, (1997,
179)): or (iii) that each of us consists of a series of substances,
changed at any break in consciousness, which pushes one towards a
constructivist account of identity through time and so towards the
spirit of the bundle theory: or (iv) even more speculatively, that the
self stands in such a relation to the normal time series that its own
continued existence is not brought into question by its failure to be
present in time at those moments when it is not conscious within that
series (Robinson, forthcoming).
(c) The ‘no-analysis’ account: The view that it is a
mistake to present any analysis. This is Foster’s view, and
Vendler (1984) and Madell (1981) appear to have similar positions.
Foster argues that even the ‘consciousness’ account is an
attempt to explain what the immaterial self is ‘made of’
which assimilates it too far towards a kind of physical substance. In
other words, Descartes has only half escaped from the
‘ectoplasmic’ model. (He has half escaped because he does
not attribute non-mental properties to the self, but he is still
captured by trying to explain what it is made of.)
Foster (1991) expresses it as follows:
…it seems to me that when I focus on myself introspectively, I
am not only aware of being in a certain mental condition; I am also
aware, with the same kind of immediacy, of being a certain sort of
thing…
It will now be asked: ‘Well, what is this nature, this sortal
attribute? Let’s have it specified!’ But such a demand is
misconceived. Of course, I can give it a verbal label: for instance, I
can call it ‘subjectness’ or ‘selfhood’. But
unless they are interpreted ‘ostensively’, by reference to
what is revealed by introspective awareness, such labels will not
convey anything over and above the nominal essence of the term
‘basic subject’. In this respect, however, there is no
difference between this attribute, which constitutes the
subject’s essential nature, and the specific psychological
attributes of his conscious life…
Admittedly, the feeling that there must be more to be said from a
God’s eye view dies hard. The reason is that, even when we have
acknowledged that basic subjects are wholly non-physical, we still
tend to approach the issue of their essential natures in the shadow of
the physical paradigm. (Foster 1991, 243–5)
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