Life (Stanford Encyclopedia of Philosophy)

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Life
First published Tue Nov 30, 2021; substantive revision Fri Jan 23, 2026
Open a textbook in biology and you’ll find definition of life.
It might consist of a list of characteristics that apply to organisms,
their parts, their interactions, or their history. Some definitions
may go beyond mere descriptions, but provide more controversial
theoretical commitments.
Like any basic concept, life is difficult to define. Most people avoid
the issue. They may ignore marginal cases, accept fuzzy boundaries, or
declare the whole issue beyond their scope. But many people believe
their work requires a rigorous demarcation of life. So, a definition
may be helpful in new scientific contexts, such as astrobiology,
origins of life, or synthetic biology. As such, the nature of life
continues to be debated.
This article focuses on the subject matter of biology: life. The first
half of this article will focus on attempts to characterize life by
both philosophers and scientists. The first section will describe
alternative accounts of definitions. Two subsections will cover
historical and contemporary definitions. Section 2 covers the recent
countertrend in skepticism toward definitions of life. Because the
various stakeholders have different goals, the second half will focus
on those goals. Sections 3, 4, and 5 cover topics that may need a
definition of life: artificial and synthetic life, the origin(s) of
life, and the search for life in the Universe. Section 6 covers
entities that are much larger or smaller than organisms. Finally,
section 7 covers the role life takes in the context of society.
1. Definition(s)
1.1 Definitions of Life from Antiquity to Darwin
1.2 Contemporary Definitions of Life
2. Definitional Skepticism
3. Artificial and Synthetic Life
4. Origin(s) of Life
5. Search For Life
6. The Macro and the Micro Perspectives
7. Ethics, Law, and Politics
8. Conclusion
Bibliography
Academic Tools
Other Internet Resources
Related Entries
1. Definition(s)
Few things in biology have been more discussed than the definition of
life. It is frustrating so little progress has been made on the topic
in the face of so much research, theory, and debate. There are many
reasons for this failure. Researchers disagree on the level of
abstraction, what to include, or even the nature of definitions
themselves. This section covers the nature and role of definitions in
the context of life.
Philosophers often seek
theoretical
definitions (also called
real
ideal
or
philosophical
definitions).
A complete theoretical definition will include individually necessary
and jointly sufficient conditions. Anything that has those will count
as that concept. Theoretical definitions can be fragile as we can
reject them with a single imagined counterexample. A classic case is
the definition of “bachelors” as “unmarried
males.” While it seems straightforward, non-bachelors fit this
definition. Consider male dogs, baby boys, widowers, etc. Similarly,
any definition of life will tend to exclude living cases or exclude
non-living cases:
Life is organized, but so are geological formations.
Life processes energy, but so does fire.
Life evolves using complex biochemistry, but so do
prions.
Life is self-sustaining, but parasites are not.
Life is at thermodynamic disequilibrium, but so is much
else.
Theoretical definitions are too rigid a standard. The real world is
far too complex for limited criteria to decide every marginal
case.
Non-philosophers are often quite frustrated by the back-and-forth that
results from theoretical definitions. Some favor
operational
(or
working
) definitions. These definitions work in practice
to narrow down the range of phenomena under consideration. This
approach is often not considered a
kind
of definition by
philosophers (see Gupta & Mackereth 2023). Operational definitions
tend to be philosophically shallow. Consider NASA’s operational
definition of life as “a self-sustaining chemical system capable
of Darwinian evolution” (Joyce 1994). It might include viruses
while excluding mules. The shallowness of an operational definition
frustrates theorists, including working scientists.
There are several other conceptions of definition, as well. The
nominal
(also
lexicographer
or
dictionary
definition is found by analyzing how people use a term. They follow
the slow process of cultural acceptance. So, such definitions will not
work to guide research on cutting edge or controversial issues.
There are also
demonstrative
or
ostensive
definitions. In such cases, we communicate concepts we can convey by
sharing observations. “That is red” while pointing at a
red block, for instance. Potter Stewart once defined pornography in
this manner by saying “I know it when I see it” (Stewart
1964). That phrase has been key to understand this kind of definition
ever since. Note that this so-called knowledge is spurious. An
internalized cultural category may
feel
as natural as a
natural kind. And people differ in terms of what they think an
ostensive definition covers. “That is gavagai” may refer
to the redness of a block, the hardness, or any other property in that
general direction. With life, scientists disagree on its use even for
objects on Earth, like viruses and prions. So ostensive definitions
are not viable for understanding life.
Then there are
stipulative
definitions, which are terms
introduced and defined by fiat. Euclidean geometry defines a circle as
a round plane figure consisting of infinite points equidistant to a
single other point. There are no possible counterexamples to this
definition, given the axioms of Euclidean geometry. This approach
provides little refuge in the real world. Consider an attempt to
define swans as “white birds with long necks.” By
stipulation, storks, great egrets, and many cranes would be swans,
while Australia’s black swans would not. Such a quick and dirty
definition seems to define the category out of hand and only works
within accepted axioms or theories. Consider that we could stipulate
life as “carbon-based reproducing entities.” Such a
definition would rule out silicon-based life by fiat. But this only
pushes the debate. It will not convince researchers find the
stipulated definition unintuitive.
The 20th century saw some steps away from definitions toward
alternative views of concepts. The most popular theories of concepts
are
prototypes
exemplars
, and
theories
(Machery 2009).
Prototype
concepts are abstract features
shared by most members of a category (Rosch & Mervis 1975; Rosch
1978; Hampton 1979, 2006; Smith 2002). The definitions of life in
biology textbooks might be a prototype concept. So, too, are the
property cluster natural kinds popular in philosophy of biology (Boyd
1991, 1999, 2010; Diguez 2013; Slater 2015).
Exemplars
are
concepts built around similarity to a particular individual case
(Medin & Schaffer 1978, Nosofsky 1986). Both prototypes and
exemplar concepts rely on similarity to some paradigm case (Komatsu
1992). The paradigmatic cases for prototypes are an imagined ideal,
while those of exemplars are real objects. For similarity-based
concepts to work in scientific cases such as life, we need an account
of which similarities matter, how much, and why.
In contrast to similarity-based concepts, theory concepts are more
nebulous.
Theory
concepts are as diverse as scientific
theories (Carey 1985, Murphy & Medin 1985, Gopnik & Meltzoff
1997). At the core, theory concepts rely on explanations for why the
members of a category share certain properties. Some theories of life
may include marginal cases of life, such as viruses, prions, or
protocells, while others might not.
In sum, there are many potential approaches to definitions. Each kind
of definition has different benefits, drawbacks, and standards of
success. Many attempts to define life have focused on either
operational and philosophical definitions. These two definitions are
at cross-purposes. Operational definitions are quick and admit
counterexamples. Philosophical definitions tend to be precise and
theoretically laden. Some disagreements dissolve after clarifying and
understanding the type of definition used. Less work has been done on
life not as a definition, but as a
concept
, such as a
paradigm or exemplar, although that is changing (Mariscal &
Doolittle 2020).
1.1 Definitions of Life from Antiquity to Darwin
This subsection explores historical definitions of life. There are
more in-depth treatments of the matter, to which an interested reader
should turn (Bedau & Cleland 2010, Riskin 2015, Mix 2018).
Approaches to this issue vary across historians, philosophers, and
scientists. That disagreement warrants caution about any individual
author’s approach.
We begin with the Greeks. In the
Phaedrus
Timeaus
and
Republic
, Plato divided life into three parts: vegetable
life, animal life, and rational life. All living creatures possessed
the first in the form of nutrition and reproduction. Animals are also
capable of sensation and locomotion. Humans have all three.
Plato’s influence in Christian theology may be apparent in
spirit if not in detail. In Christian theology, human life is not just
rational. Human life is involved an eternal, spiritual soul and an
internal, conscious life.
Plato’s student, Aristotle, had a different notion. For
Aristotle, living things had an appropriate form, material, and
goal-directedness (
De Anima
, 412a1–416b). Aristotle
held life to be a form of self-motion, perpetuation, or
self-alteration (Byers 2006). Unlike non-life, living beings have the
capacity to resist internal and external perturbations. For Aristotle,
that was the essential distinction, other features were accidental.
This project for demarcating the essential from the accidental for
life has persisted to this day.
Centuries later, Descartes drew a different distinction. Descartes
held there was a larger gap between animals and rational agents than
between inanimate objects and animals. This was a turn away from
medieval approaches, which had taken the gap between vegetables and
animals to be broader. For Descartes, animals are analogous to complex
clocks and lack the inner or spiritual life central to the human
experience (Descartes 2010/1664). Descartes’ category of life
neither mapped onto Greek conceptions nor current conceptual
frameworks. Scholars who agreed with Descartes were dubbed
‘mechanists.’ Many regard mechanistic thought as
continuous with current science, but this is anachronistic. Few
current scientists accept the theoretical underpinning central to
mechanistic thought.
There were many responses to Descartes throughout the next three
centuries. ‘Vitalism,’ the collective label for these
responses, was a heterogenous philosophical position. The only common
feature was the adherents’ doubt of a mechanistic view of life.
Vitalists took the defining features of life as any of immaterial
causes, particular arrangements of matter, a special life fluid, a
particular end goal, or even mental forces. Students sometimes learn
about Friedrich Wöhler’s synthesis of urea from ammonium
cyanate. A cartoon version of history has that event as the ultimate
death knell of vitalism, though the view persisted for decades
afterward. In that story, if biological chemicals can be produced from
mere chemistry, then biology is also mere chemistry. Although this was
an important step, history rarely has such pivotal moments. Many
chemists already had accepted a mechanistic world view and some
researchers continued to develop vitalist theories well into the 20th
century (Bergson 1959, Driesch 1905/1914).
1.2 Contemporary Definitions of Life
The 20th century saw the mechanist/vitalist divide dissipate. But
without a vitalistic understanding, giving an account that
distinguishes life from non-life is more difficult. In the past
century and a half, hundreds of scientists, philosophers, and others
have tried their hand at defining life. Many were motivated by new
science and technologies, such as artificial life, synthetic biology,
origins of life, and astrobiology. Research in these areas complicate
the issue by violating some of the traditional groupings of properties
associated with life.
There are many books, articles, and workshops on the nature of life
(Pályi et al. 2002, Popa 2004, Bedau & Cleland 2010).
Popa 2004
Trifonov 2011
Malaterre & Chartier 2019
Matter & Energy
Mechanistic
: pragmatic interpretations that see life as
a complex machine, including thermodynamic approaches
Energy
: relating to terms like
force
Material-Related
: those based on biochemistry and other
feature of life on Earth
Matter
: relating to terms like
organic
material
, and
molecules
1. Matter/Energy
, including the categories:
Holistic Definitions
: function- and purpose-related
descriptions that treat life as a collective property
System
: relating to terms:
systems, organization,
organism, order, network, etc.
1a. Metabolism
: including digestion, fermentation,
digestion, and thermodynamics
Chemical
: relating to terms:
process
metabolism
reactions
, etc.
1b. Catalysis and Synthesis of Proteins:
including
everything from monomers to macromolecules
Structure
Reductionist
: definitions which focus on underlying
structures common to life
2. Structural
, including the single subcategory:
Cellularist:
views of life that take single cells to be
the relevant origin and, hence central feature of life
2a. Cellular/Structural Features
: including cell
division, stressors, and transporters
Environment
Environment
: relating to terms:
external
etc.
3. Environmental Interactions
, a broad category that
included:
3a. Micro/Macro Environment
: including all sorts of
mutualisms and properties for interacting with other creatures
3b. Plant/animals related
: including those intersecting
with human society: ticks, farming, spillover diseases, etc.
3c. Human related
: including phenomena that resemble
human physiology or produces immune responses, as in humans
Evolution
Evolution
: relating to terms:
evolve
change
mutation
, etc.
4. Evolution
, including the single subcategory:
4a. Evolvability
: including most features of heredity
and evolution, such as variation, adaptation, and speciation
Information
Minimalist
: approaches that use the least amount of
information to demarcate life from non-life
Complexity:
relating to terms:
complex
information
, etc.
5. Information
, including the single subcategory:
Genetic
: views of life that take replication and
variability to be the origin and key feature of life
Reproduction
: relating to terms:
reproduce
replication
, etc.
5a. Genetics:
including all genetic material,
transcription and translation, and subsequent epigenetic
modification
Miscellaneous
Cybernetic
: approaches to life that abstract in such a
way as to incorporate computer-based artificial life
Ability:
relating to terms:
ability
capacity
, etc.
Generalist
: approaches that are broad, obscurantist, or
otherwise vague
Vitalist
: definitions that take life to be an as-yet
mysterious force, organization of matter, or other phenomenon
Parametric:
definitions that identify one or more
relevant features of life
Table 1.
Some recent attempts at
meta-categories for life definitions. Each column is one
account’s categories, the rows are lined up according to rough
similarity.
There are perhaps thousands of competing definitions proposed across
hundreds of articles. A true survey of that variety would be beyond
the scope of this article and beyond your patience as a reader.
Nevertheless, some broad categories have been proposed that might
offer some insight into current contending definitions. Table 1
summarizes three of the most rigorous attempts this century to
categorize definitions of life.
Each of these authors used different approaches to arrive at their
categories. Popa 2004 and Trifonov 2011 attempted to reverse engineer
the categories from dozens of definitions collected from many dozens
of experts, while Malaterre and Chartier 2019 conducted a more
extensive, text-mining approach across 30,000 scientific articles
selected from journals that published pieces in biology. As one can
see, there are areas of rough overlap, but each categorization scheme
has its own unique categories as well.
Most of the definitions considered by these authors straddle some of
these distinctions and are often ambiguous as to whether they are
intended to be theoretical definitions, operational definitions, or
something else.
Categorizing involves making choices and reasonable people can
disagree about whether each belongs in one or more categories. It is
best to see a category scheme (or meta-scheme) as the beginning of a
conversation rather than the end. The takeaway from current
understandings of the definition of life is that there is no consensus
forthcoming. One concern is that these are summaries of attempts to
define a category for which there is only loose agreement. Many
scientists disagree as to the phenomena a definition of life is
intended to unify. Some theories would include prions, viruses, and
entities only hypothesized to exist in the origin of life. Others seek
a theory that excludes them. Some might accept digital organisms as
alive, others would deny this approach. Nor can we ignore the
question, as conceptual equivocation has significant costs for
research in both time and money (Trombley and Cottenie 2019). Given
the diversity described above, one may wish to adopt a
definitional pluralism
: there are many ways to be alive. For
some reason, that approach is not common in the literature.
2. Definitional Skepticism
One may see the previous section as a kind of conceptual tug o’
war. Perhaps that ought to lead us to be skeptical in some way of the
project of defining life. In the presence of rampant disagreement
between experts, there are several ‘moves’ available (e.g.
Kelley 2024):
Advocacy: find or create a view and advocate for it
Next-best theory: find the theory that accounts for the
‘most’ phenomena that overlap across accounts (cf.
Sterelny 1993, p. 83)
Stipulate: define a new term of art
Specify: divide the concept into distinct types and label
them
Polysemy: accept a concept that is vague and slippery among
uses
Skepticism: hold that we may never get an answer and move on with
a lessened project (c.f. Cleland 2019).
Eliminativism: disregard the concept of life altogether and
reformulate the issue (e.g. Jabr 2013)
Most researchers agree there is a distinction between life and
non-life. It is often understood as a difference in kind rather than
one of degree. Furthermore, most accept that life is a natural kind,
rather than a human concept. That said, a recent theme has been to
express skepticism of life definitions as a goal. The literature on
the definition of life is vast, repetitive, and utterly inconclusive.
Philosophers disagree about the source of the lack of consensus. They
cite unstated assumptions in either the definer’s approach or
the question itself. Many scientists are less skeptical of the goal of
defining life, albeit more resistant to engaging in the philosophical
debate.
Some argue that any definition of life presumes a theory of life
(Cleland and Chyba 2002, Benner 2010, Cleland 2019). Though unstated,
this view is akin to the theory-theory of concepts, described in
section 1. a common analogy is to early chemical theory. According to
this analogy, early alchemists likened the alchemists’
Aqua
regia
(“royal water”) and
Aqua fortis
(“strong water”). Development of atomic theory revealed,
Cleland argues, that the true nature of water was H
O,
while the other ‘waters’ were HNO
+ 3 HCl and
HNO
, respectively. Cleland advocates avoiding definitions
altogether, fearing they will blind us to new instances of life, and
instead opts for tentative criteria, which she believes avoid the
implicit dogma of even operational definitions.
Other authors have pointed out that the explanandum of life is itself
up for debate (Tsokolov 2009, Mix 2020, Parke 2020). According to
Emily Parke, some accept life as applying to individuals, whereas
other definitions apply to collectives first (including entire
planets) and individuals derivatively. Most believe life is some kind
of entity rather than some kind of relation or process (but see
Nicholson and Dupré 2018). Parke also points out that some
definitions seek a material basis, perhaps limiting life’s
substrate to the biochemistry we know on Earth, while others are
functional. Sagan worried about biochemical definitions because they
were prone to ‘Earth Chauvinism’ for privileging our own
biochemistry (1970). Other authors take our biochemistry to be
justified as universal (Pace 2001, Benner et al. 2004, but see Bains
et al. 2024). Finally, Parke distinguishes between those that seek
clean boundaries and those that accept the possibility of a continuum
of ‘lifelikeness.’
Other authors have advocated a kind of quietism about definitions,
maintaining that folk concepts need not match up with scientific ones
(Machery 2011), any definitions would not change scientific practice
(Szostak 2012), advocated a radical conceptual rethinking (Mariscal
& Doolittle 2020), or even denied the distinction between life
& non-life (Jabr 2013).
This last position of eliminativism could be expanded as it helps
illustrate all other life skeptical positions. Cowie 2009 classifies
eliminativist goals as either linguistic or ontological. Ontological
eliminativists don’t believe the objects they are eliminating
exist. We’re all eliminativists about something, perhaps ghosts
or fairies. Linguistic/conceptual eliminativists, on the other hand
are suspicious of theoretical terms or concepts, what Ramsey 2020
calls ‘category dissolution’ or ‘conceptual
fragmentation.’ In essence, it’s not that there
aren’t living things, it’s just that the category
life
is heterogeneous rather than a natural kind. According
to Cowie, one can deny that anything matching our theoretical
definition of life exists in the world while still accepting it as a
useful fiction. One may also think scientific theories about life are
fruitless or that the term is too vague and confused to be useful,
without doubting life exists. If we accept any of these alternatives,
we should perhaps avoid ever using the term ‘life’ in
isolation and instead reference Metabolic Life and Evolutionary Life
and all the other conceptions.
At play in these various forms of skepticism are several underlying
assumptions. Among other disagreements, researchers disagree about
what life is, whether it is a natural kind with an essence or a human
construct; they disagree as to the purpose of defining life if it will
not change scientific practice; and they disagree as to the features
of life that are relevant and the ones that are mere consequences.
When researchers hold unstated assumptions such as these, they are
liable to mistake the source of their disagreement.
3. Artificial and Synthetic Life
The rest of this article will focus on uses of the various life
concepts. The concept of life is central to several scientific and
societal purposes. Several definitions described above originated
among theorists working on these applied issues. This section focuses
on artificial and synthetic life.
In principle, most contemporary scientists and philosophers believe
life can be created. But researchers disagree as to how that would
work. In functional approaches, recreating the formal organization of
organisms may be enough. Complexly configured robots
(“hardware”) or computer programs (“software”)
might qualify. This view is known as Strong Artificial Life (Strong
A-Life for short). Strong A-Life has received much of the same
pushback as the Strong Artificial Intelligence approach before it
(Sober 1991, Boden 1999, Brooks 2001). Those who reject Strong A-Life
hold that functional approaches miss the essential features of
biology. Epistemic objections hold that we lack the relevant
biological knowledge to recreate it in a digital framework. Most of
the objections to Strong A-Life have been ontological. Such objections
hold that representations cannot be equivalent to what they represent
or that life requires chemical embodiment. An ontological objection to
Strong A-Life rules it impossible by fiat.
Weak A-Life approaches, on the other hand, don’t presume the
ontological equivalence of structurally similar circuits and cells.
Instead, proponents suggest the more modest goal of developing a
deeper understanding of life as we know it by exploring the effects of
various parameters in simulations, placing life in a broader context
of possible biology (Langton 1989, 1995). For example, in the Terra
program, software was pitted against other software for processing
power (Ray 1993). Unexpected by the researchers at the time, software
parasitism evolved: software would co-opt the reproductive processing
of other software. Policing mechanisms also evolved, leading to an
arms race between free-riders and the software trying to stop
them.
Whether one accepts the strong or weak interpretation of A-Life, these
in silico
approaches are cheaper than equivalent work done in
real organisms. They also offer possibilities that are not available
in ordinary biology, such as programming alternative parameters to
take the place of laws of nature and exploring relationships across
deep time and space.
Another approach worth highlighting is that of synthetic life
(“wetware”). Synthetic life can also address some
questions of A-Life, while allowing for a finer grain of realism.
Synthetic life approaches have explored creating self-replication
(Lincoln & Joyce 2009), minimal genomes (Koonin 2000, Hutchinson
et al. 2016), chemical evolution (Gromski et al. 2020), synthetic
living machines or ‘xenobots’ (Raman 2024), a
‘xenobiology’ with different genetic bases (Aparicio
2025), and other projects. Artificial intelligence approaches have
catalyzed this research, producing what some researchers call a
looming deluge (Groff-Vindman et al. 2025). Not all synthetic biology
is in the business of investigating life as it could be, as not all
computer programming is A-Life. Still, the tools developed by both can
be illuminating. By exploring possibilities, scientists can discover
hidden relationships, revealing which aspects of life are more or less
plausible than expected.
4. Origin(s) of Life
The nature of life is inextricable from its origin. Ancient and modern
thinkers accepted that life often arose spontaneously from non-life.
Two centuries of experiments and debates overturned this view. Louis
Pasteur’s abiogenesis experiments illustrated that life does not
typically come from non-life. That insight, however, made Life’s
origin one of the biggest and most important puzzles in science.
Darwin said little about the problem. His one speculation came in a
letter to his friend Joseph Hooker. Darwin confided that he imagined
life originating in “some warm little pond” (see Other
Internet Resources below; and Peretó et al. 2009). Work on the
subject was sparse until the 1920s. In the interbellum
period,Alexander Oparin and J.B.S. Haldane separately proposed
hypotheses for life’s origin (Haldane 1929; Oparin 2010/1936).
As a graduate student in the 1950s, Stanley Miller tested the
proposal, discovering dozens of amino acids in the mixture (1953).
Since then, the field of origins-of-life studies has expanded
dramatically.
Our earliest reliable records of this planet, some 3.5 billion years
ago, contain evidence of microbial fossils. These include shapes
distinct of current prokaryotes, as well as carbon-ratios distinctive
to life as we know it (Schopf 1993, Schopf et al. 2017). Analyses have
pushed our confidence in life’s earlier origin further
back(Rodriguez et al. 2024). In short, as soon as Earth was not
molten, it was filled with life (Pearce et al. 2018, Lineweaver 2020).
How life started and why it started so quickly remains one of the most
pressing open questions in science.
There are many open philosophical issues in origins of life research.
The origin of life sounds like one question, but it is not.
Origins-of-life researchers may research how life
could have
originated, how it
did
originate (Scharf et al. 2015,
Mariscal et al. 2019). They may focus on historical adequacy, natural
explanations, or similarity to life as-we-know-it (Malaterre et al.
2022). Some steps in the process could have been chancy, others could
have been contingent, still others could have been the only way life
ever originates anywhere in the Universe.
There are several broad approaches to investigating the origin of
life. “Bottom-Up” approaches begin with pre-biotic
chemistry and explore how it could withstand stressors in order for
lifelike entities to form and evolve. At present, there are many
unsolved problems, most notably that most energetically favorable
interactions would consume the proto-life forms involved. Scientists
have attempted to ease the problem by relaxing assumptions: perhaps
the environment provided our first boundaries (Koonin 2009), or
perhaps it provided proto-genetic material (Mathis et al. 2017), all
of this could have occurred in a viscous solvent instead of a cell (He
et al. 2017), or on a surface (Wächtershäuser 1988), or
using a variety of entities that eventually became encapsulated (Eigen
& Schuster 1977). Nevertheless, the gulf between the pre-biotic
chemistry and the simplest life forms is still huge and any number of
explanations only account for a tiny portion of the conceptual
distance.
Another approach, “Top-Down,” uses current taxa to infer
the nature and timing of the origin of life on Earth. To do so, we
take current examples of life on Earth and trace their ancestry, by
comparing the nearly hundred shared genes associated with biological
translation (Koonin 2011). All life shares a last universal common
ancestor, “LUCA” for short. There may have been several
origins of life, but our evidence is insufficient to distinguish this
scenario from a single origin. Nevertheless, at least one origin in
Earth’s pre-biotic conditions led to the existence of LUCA. This
is an important constraint upon theorizing about the origin of life.
LUCA was but one creature in a larger population and existed long
after the origin of the first organism. There are also a variety of
concerns with respect to LUCA: whether it was simple or complex
(Mariscal & Doolittle 2015); whether it had a membrane that
resembled any of the current membranes (Koonin 2011); whether the
genes it contained were ancestors of our own genes or subsequently
acquired (Doolittle & Brown 1994, Woese & Fox 1977; Woese
1998); whether its genome was made of DNA (Forterre 2006a), whether
its metabolism was a niche for other microbes or (Moody et al. 2024);
and where and when it lived.
The gap between Top-Down and Bottom-Up approaches is huge. There were
untold generations passed between pre-biotic chemistry and LUCA. We
may never be able to solve Life’s origin, but each step brings
us closer to understanding the trajectory.
5. Search For Life
Even the most pessimistic analyses of the likelihood of life suggests
life on Earth is not unique (Frank & Sullivan 2016). Many
scientists take that as a good reason to search for life elsewhere in
the Universe. The current search for life elsewhere focuses on two
extremes: chemical byproducts of life and technological signals of
intelligent
life. The biology of alien populations might be
interesting, but they are hard to study from a distance. Thus, we
search for biosignatures that might identify life from a great
distance. We’ll take each in turn.
Biosignatures, as the name implies, are markers of life. Chemical
biosignatures are compounds rarely produced by mere chemistry.
Biosignatures imply a material conception of life, likely in the form
of biochemistry, metabolism, or thermodynamics. There have been many
attempts to detect biosignatures, most often on Mars. These approaches
include experiments done on planetary surfaces, observations from
Earth or low-earth orbit, and study of meteors and other debris from
nearby planetary bodies.
Satellite or telescope observations of other planets have been used to
search for gasses outside of thermodynamic equilibrium. Methane has
been sporadically detected on Mars since 2004 (Formisano et al. 2004,
Webster et al. 2018) with an accompanying claim of formaldehyde
detection (Peplow 2005). The James Webb Space Telescope can image
exoplanets at resolutions allowing the detection of gas biomarkers in
the atmospheres of exoplanets (Loeb & Maoz 2013, Seager et al.
2025). Exoplanets will continue to be a source of attention, as
putative biosignatures continue to be debated (e.g. Madhusudhan et al.
2025). A biosignature is a negative proof: one must rule out all
abiotic possibilities. As such, it will always remain open whether we
have not considered every potential reaction and circumstance that
allows non-living processes to produce molecules typical of life
(Malaterre et al. 2023).
By contrast, there have been scarce attempts to detect chemical life
while on the surface of another planet. In 1975, NASA sent the Viking
landers to Mars, tasked with a variety of scientific experiments
including some that were purported to detect life if it was present.
One, the Labeled Release Experiment, did, but its results were
inconsistent with the other on-board experiments, so the result was
deemed inconclusive (Levin & Straat 1976, Ezell & Ezell 1984).
The current
Perseverence
rover on Mars is able to assess
certain biosignatures and upcoming missions by NASA, the Chinese
National Space Administration, and the Japanese Aerospace Exploration
Agency all seek to determine whether Mars has evidence of past or
current life.
It is not obvious that life on Mars would be a separate origin than
life on Earth, as the two planets exchange tons of rocks each year and
it is at least theoretically possible that life could have formed on
one planet and been subsequently transferred to the other (McKay
2010). Since Mars is a smaller body than Earth, it coalesced before
Earth and thus it is conceivable that life might have formed there
first, although this is a marginal view in the astrobiology community.
Meteorites from Mars and other planetary bodies have also been the
source of purported biosignatures. The Martian meteorite ALH84001 was
instrumental in forming the science of astrobiology in 1996, after
NASA scientists discovered bacteria-like structures (McKay et al.
1996). Subsequent meteorites have also garnered scientific interest
(e.g. White et al. 2014).
The other major attempt to search for life, that of searching for
intelligence, more readily captures the imagination. A famous instance
is Percival Lowell’s drawings of Martian canals in the 1890s.
Influenced by the mid-1800s observations of apparent channels
crisscrossing Mars, Lowell drew a series of canals based on his
observations. Science fiction soon picked up the observation and
conjectured a dying civilization, hoping to squeeze water out of the
last bits of remaining ice in the Martian poles (Wells 1898).
The early 20th century saw increased interest in detecting radio
signals from Outer Space. This interest accelerated after the launch
of Sputnik in 1957. SETI research has not been publicly funded since
1994, but private and public donors, as well as academic and lay
researchers have kept the program going since. There are many
technical challenges to the search: space is unimaginably huge,
signals are weak, possibilities of interstellar communication are
myriad, and our searches can only cover an insignificant portion of
the task. Still, there is continued interest in discovering
technosignatures and increasingly precise ways of theorizing and
observing them (Haqq-Misra 2024).
More controversially, many dozens of messages have been sent into
Outer Space since 1974. A few have been in the form of physical
objects aboard spacecraft, but most have been radio signals aimed at
promising stars or star clusters. sometimes called Active SETI or METI
(Messaging Extraterrestrial Intelligence). Although the practice
continues due to its low cost and relative ease, many philosophers,
scientists, and policy experts have come out against the practice due
to the risk of broadcasting our presence to potentially hostile forces
on behalf of future generations that cannot consent (Smith 2020).
6. The Macro and the Micro Perspectives
Scientists are more concerned about philosophical questions when they
become aware of scientific limitations or their conceptual choices.
Scientists who study deep time, deep space, abstract issues, or
questions of ethics are often aware of the philosophical choices that
influence their research. Every step from identifying research
questions to interpretation of data to application is fraught (Douglas
2000). This section goes through other scientific contexts in which a
concept of life is relevant below and above the organism level.
There are several biological entities for which it is an open question
as to whether they are alive. Viruses, for example, are units of
genetic material encased in a protein coat. It is unknown whether all
viruses share common ancestor (Koonin et al. 2006, Moreira and
López Garcia 2009). We also do not know how they originated, be
it escaped transposable elements, reduced cells, or some ancestral
third option (Forterre 2006b). The status of viruses as living is
mired with controversy, with some people holding virons to be alive,
others believing them not to be, and a third camp has them as living
only in the context of an infected cell, but a mere ‘seed’
otherwise (Forterre 2010).
There are other entities in the “twilight zone of
microbiology.” These include transposable elements, viroids,
unculturable (but existing) microbes, organisms in vegetative states,
and prions (Postgate 1999). The problems facing each of these are
similar: they have some, but not all properties associated with life.
For example, prions are protein products of life that can fold other
prions in a way that allows for cumulative evolution (Li et al. 2010).
They are rarely included in the category ‘life’ due to
their inactivity in most settings and rather simple nature.
If there is a twilight zone of microbiology, there is also a twilight
zone of ecology. Organisms form populations, species, lineages,
clades, and ecosystems. The status of each of these is an open
question, but they have many of the same features associated with life
as described above. Perhaps the strongest case can be made for
eusocial insects, such as some ants, bees, wasps, and termites. In
several species, there are rigid distinctions between the castes that
reproduce and those that do not, with many of the latter serving the
role of caring for the young (Hölldobler & Wilson 2009). One
might note that entities above the organism level are as a rule less
integrated and connected than the organisms that comprise them. From
the perspective of every item in the biological hierarchy, its parts
are much more homogenous than it is. Our cells seem much more
integrated and self-contained than our bodies. So, too, are individual
insects more self-contained than the colonies to which they
belong.
Most controversial has been the case of Gaia. Gaia is a term from
Greek mythology; she is a personification of the planet Earth. In
1979, James Lovelock, revived the concept in his book,
Gaia: A New
Look at Life on Earth
. In his view, the Earth-wide set of
interlocking ecosystems could be viewed as a single entity. One
insight of Lovelock’s was already mentioned in the previous
section: planet-wide interactions are the scale that matters in
detecting life elsewhere. Lovelock’s book sparked controversy
centered around the plausibility of his model of the Earth as a
self-regulating homeostatic system. In the view of many at the time,
it was an inaccurate description: Earth could not evolve in principle,
and the subsequent ontological move of granting Earth the status of
life was unmotivated (Doolittle 1981). Recent attempts to revitalize
the notion of Gaia on a more theoretic footing involve both abiotic
and biotic regulatory mechanisms and natural selection acting at the
level of clades (Lenton et al. 2018, Doolittle 2025). Regardless of
current attempts at a theoretical justification, the thought of Earth
as a living entity motivated many in the environmental movement and
the idea remains a common reference.
7. Ethics, Law, and Politics
The term ‘life’ is important outside of science. Often,
the focus is the beginning or end of individual lives. The start of an
individual life has been the source of contraception and abortion
discussions (Noonan 1967, Dellapenna 1978). The end of individual
lives was also a heated debate in the 20th century as new technologies
were able to keep human bodies alive long after they would have died
in nature (DeGrazia 2016). It remains controversial in cases of
euthanasia, or voluntary death to relieve pain and suffering.
We should begin by distinguishing between life and other phenomena
that are often conflated with it. In public discourse, the existence
of life is often conflated with its value. But we can distinguish
between life and sentience, personhood, and moral considerability.
It’s unclear how much metaphysical, epistemic, or moral weight
the category of life has independent of other properties. Although
many people seem to imply that life brings with it a kind of moral
considerability, practice belies this. Most humans don’t mind
killing bacteria for the sake of cleanliness and many eat or wear the
flesh of animals. So, in many discussions, life is only valuable when
it is the vehicle for other nebulous properties that confer value like
sentience, personhood, moral considerability, or even immaterial
souls.
Attributing moral worth to non-living entities is still a minority
view in environmental or comparative philosophy (but see Leopold 1949,
Basl 2019). Thus, a starting position might be that life is a
prerequisite for sentience and sentience is a prerequisite for moral
considerability (Birch 2024). Attribution of life relies on a concept
of life, which means both are contested.
If any living entities have a distinct moral or ontological status,
most philosophers accept that humans are among them (Rolston 1975,
Goodpaster 1978). In these contexts, it matters when individual humans
come into being and acquire such a status in their own right, be it
conception, birth, or some time period in between. Unfortunately,
developmental biology does not provide an uncontroversial starting
point for when ‘life’ begins (Maienschein 2014). Still,
policy makers and medical practitioners draw lines with respect to the
possibility of self-sustained life, sentience, or other features.
There is still a pro-life/pro-choice split in cultural politics, which
is somewhat lessened in European countries (Corbella 2020). When
broken down into issues, most people are pro-choice with respect to
cases of birth defects, women’s health, or cases of rape. Still,
only about 15 percent of respondents are consistently pro-life across
all scenarios (Osborne et al. 2022). There are equivalent, but less
tendentious analogues in the contexts of euthanasia, the death
penalty, war, and the prevention of death and disease. In these
debates, both ethical and metaphysical commitments matter.
Questions of life are often raised by new technologies. In the
abortion discussion above, for example, new techniques to end
pregnancies, from birth control to abortion procedures, as well as new
medical technologies facilitating premature deliveries made the topic
more contentious. Current bioethics research in artificial wombs
complicates many of the traditional issues with abortion (Kendal
2025). Even if a fetus were able to be brought to term artificially,
the process would still require a medical treatment and genetic
parenthood, not to mention the costs of childrearing and questions of
gestational choice (Charles 2024).
Other technological innovations also raise questions about life. One
such area is that of transhumanism (c.f. More and Vita-More 2013).
Transhumanism is the movement aimed toward the use of technology for
the human enhancement of social, psychological, and physical lives.
These can range from prosthetics to implants or from pharmaceuticals
to mental ‘uploading.’ There are bioconservatives, who
argue against transhumanism for practical, moral, or aesthetic
reasons. There are also posthumanists, who look forward to a world in
which humans are replaced or eliminated by subsequent artificial
intelligence. The debate over whether such posthumans might be
‘alive’ is similar in structure to the artificial life
discussion in section 3. Bioconservatives also argue against this
view. Among the topics in these debates are whether a particular
technology counts as therapy or enhancement, whether the risks of
alteration outweigh the benefits, whether certain goals of
transhumanism are even possible, and which alterations will affect the
moral or ontological status of the people that receive them.
That life is a source of ethical, legal, and political controversy is
to be expected. It is beyond the scope of this article to adjudicate
these debates. Still, advocates ought to be aware of the deep
vagueness and disagreement about life within philosophy and biology.
There is minimal consensus with respect to what life is, what an
individual living organism is, when individual lives begin or end, and
what features of life ground moral considerability. One ought not
appeal to biology as a ground for their moral views, lest they wish
their views to be vague and inconclusive.
8. Conclusion
Although the conceptual terrain of life concepts is well covered,
there is no accepted view of it. Given the disciplinary backgrounds,
explanatory values, and theoretical commitments of the stakeholders
involved, this is unlikely to change. Still, a wide range of practices
rely on competing conceptions of life. As described above, these
include artificial life, origins-of-life research, the search for
life, and ethics, and politics.
Future scientific discoveries or inventions may break this impasse..
Consider the development of atomic theory, as discussed in section 2.
Atomic theory created new divisions that scientists accepted as more
real than the categories of the ancients or alchemists. With this
conceptual fragmentation, scientists discarded old categories and
accepted new ones. One can imagine something similar happening in the
case of life. Many discoveries might show a clear cluster of how
complex, lifelike entities can form from prebiotic chemistry. These
could win over the majority of the scientific and philosophical
community (e.g. Weber 2007, 2010).
Conversely, scientific communities could simply decide based on shared
values or explanatory goals. The example of death may be illustrative.
After decades of debate, physicians did not make a decision, not a
discovery. Physicians concluded the irreversibility of death was the
most important property for their purposes. They adopted the concept
of whole-brain death as their operational criterion (DeGrazia 2016).
The facts on the ground did not change, but the shared understanding
did.
Finally, life could be accepted as a polysemous concept with each
definitional cluster applying to a subset of the whole. These might
include biochemical life, evolutionary life, metabolic life, etc.
Researchers may rely on context, accept miscommunication, or stipulate
the kind of life they mean. The example of planets, discussed in
Brusse 2016 may help make this point. There was always a huge
diversity within the category planet, which included the Sun and Moon
until the Renaissance. In the early 1800s, asteroids were discovered.
At first, they were considered planets. They were demoted to
‘minor’ planets a few decades later, then simply
‘asteroids’ after the 1950s. Pluto was discovered in 1930,
recognized as the smallest planet by the 1970s. Then, from 1992 until
2006, many objects similar to Pluto were discovered. Finally,
astronomers redefined the term ‘planet.’ Now a scientific
‘planet’ covers two distinct, but interesting categories:
typical planets and dwarf planets. Similarly, perhaps some of the
categories described in section 1.2 will form the basis of accepted
sub-categories of life.
It is still an open question as to how long the current situation will
persist. We may be days or decades away from a discovery or decision
that forces a scientific reckoning. For now, the debate continues.
Bibliography
Aparicio, Alberto, 2025, “Accept No Limits: Biocontainment
and the Construction of a Safer Space for Experimentation in
Xenobiology as a Legacy of Asilomar,”
BioSocieties
20(1): 49–72.
Aristotle,
De Anima
(selections), in M.A. Bedau &
C.E. Cleland (eds.),
The Nature of Life: Classical and
Contemporary Perspectives from Philosophy and Science
, New York:
Cambridge University Press, 2010, pp. 9–14.
Bains, William, Petkowski, Janusz J. and Seager, Sara, 2024,
“Alternative Solvents for Life: Framework for Evaluation,
Current Status, and Future Research,”
Astrobiology
24(12): 1231–1256.
Basl, John, 2019,
The Death of the Ethic of Life
, Oxford:
Oxford University Press.
Bedau, Mark A., and Carol E. Cleland (eds.), 2010,
The Nature
of Life: Classical and Contemporary Perspectives from Philosophy and
Science
, New York: Cambridge University Press.
Benner, Steven Albert, 2010, “Defining life,”
Astrobiology
, 10(10): 1021–1030.
doi:10.1089/ast.2010.0524
Bergson, Henri, 1907 [1959],
L’évolution
créatrice
, Paris: Presses Universitaires de France.
Birch, Jonathan, 2024,
The Edge of Sentience: Risk and
Precaution in Humans, Other Animals, and AI
, Oxford:
Oxford University Press.
Boden, Margaret Ann, 1999, “Is metabolism necessary?”
The British Journal for the Philosophy of Science
, 50(2):
231–248. doi: 10.1093/bjps/50.2.231
Boyd, Richard, 1991, “Realism, Anti-Foundationalism and the
Enthusiasm for Natural Kinds,”
Philosophical Studies
61(1–2): 127–148.
–––, 1999, “Homeostasis, species, and
higher taxa,” in R. Wilson (ed.),
Species: New
Interdisciplinary Essays
, Cambridge, Mass.: MIT Press, pp.
141–185.
–––, 2010, “Homeostasis, Higher Taxa, and
Monophyly,”
Philosophy of Science
, 77(5):
686–701.
Brooks, Rodney, 2001, “The relationship between matter and
life,”
Nature
, 409(6818): 409–411
Brusse, Carl, 2016, “Planets, pluralism, and conceptual
lineage,”
Studies in History and Philosophy of Science Part
B: Studies in History and Philosophy of Modern Physics
, 53:
93–106.
Byers, Sarah, 2006, “Life as ‘Self-Motion’:
Descartes and ‘the Aristotelians’ on the Soul as the Life
of the Body,”
The Review of Metaphysics
, 59(3):
723–755.
Carey, Susan E., 1985,
Conceptual Change in Childhood
Cambridge, Mass.: MIT Press.
–––, 1988, “Conceptual differences between
children and adults,”
Mind & Language
, 3:
167–181.
Charles, Sonya, 2024, “At What Price? Abortion versus
Artificial Womb,”
IJFAB: International Journal of Feminist
Approaches to Bioethics
, 17(2): 123–141.
Cleland, Carol E., 2019,
The Quest for a Universal Theory of
Life: Searching for Life as we Don’t Know It
(Vol. 11),
Cambridge: Cambridge University Press.
Cleland, Carol E. and Christopher F. Chyba, 2002, “Defining
‘life,’”
Origins of Life and Evolution of the
Biosphere
, 32(40): 387–393.
Corbella, Mercedes, 2020, “Globally, seven in ten adults
favor allowing abortion,” IPSOS, published online 17 August
2020.
Cowie, Fiona, 2009, “Why isn’t Stich an
Eliminativist?” in D. Murphy and M. Bishop (eds.),
Stich:
And his Critics
, Chichester: Blackwell Publishing, pp.
74–100.
DeGrazia, David, “The Definition of Death,”,
The
Stanford Encyclopedia of Philosophy
(Spring 2017 Edition), Edward
N. Zalta (ed.), URL =
>.
Dellapenna, Joseph W., 1978, “The History of Abortion:
Technology, Morality, and Law.”
University of Pittsburgh Law
Review
, 40: 359–428.
Descartes, René, 1664 [2010], “Treatise on
Man,” in M.A. Bedau and C.E. Cleland (eds.),
The Nature of
Life: Classical and Contemporary Perspectives from Philosophy and
Science
, New York, N.Y.: Cambridge University Press, pp.
15–20; original translation by J. Cottingham, R. Stoothoff, and
D. Murdoch,
The Philosophical Writings of Descartes
Cambridge: Cambridge University Press; original work published in
1664.
Diguez, Antonio, 2013, “Life as a homeostatic property
cluster,”
Biological Theory
, 7(2): 180–186.
Doolittle, W. Ford, 1981, “Is Nature Really Motherly?”
CoEvolution Quarterly
, 29: 58–63.
–––, 2025, “Darwinizing Gaia: Conceptual
Approaches,”
Philosophical Transactions B
380(1931): 20240089.
Doolittle, W. Ford and James R. Brown, 1994, “Tempo, Mode,
the Progenote, and the Universal Root,”
Proceedings of the
National Academy of Sciences
, 91(15): 6721–6728.
Douglas, Heather, 2000, “Inductive Risk and Values in
Science,”
Philosophy of science
, 67(4):
559–579.
Driesch, Hans, 1905 [1914],
The History & Theory of
Vitalism
, C.K. Ogden (trans.), London: Macmillan and Co.,
Limited.
Eigen, Manfred and Peter Schuster, 1977, “A Principle of
Natural Self-Organization,”
Naturwissenschaften
63(11): 541–565.
Ezell, Edward Clinton and Linda Neuman Ezell, 1984,
On Mars:
Exploration of the Red Planet, 1958–1978
(NASA History
Series: Volume 4212), Washington, DC: National Aeronautics and Space
Administration.
Fields, Chris and Levin, Michael, 2025, “Life, its Origin,
and its Distribution: A Perspective from the Conway-Kochen Theorem and
the Free Energy Principle,”
Communicative & Integrative
Biology
, 18(1): 2466017.
Formisano, Vittorio, Sushil Atreya, Thérèse
Encrenaz, Nikolai Ignatiev, and Marco Giuranna, 2004, “Detection
of methane in the atmosphere of Mars,”
Science
306(5702): 1758–1761.
Forterre, Patrick, 2006a, “Three RNA cells for ribosomal
lineages and three DNA viruses to replicate their genomes: a
hypothesis for the origin of cellular domain,”
Proceedings
of the National Academy of Sciences
, 103(10):
3669–3674.
–––, 2006b, “The origin of viruses and
their possible roles in major evolutionary transitions,”
Virus Research
, 117(1): 5–16.
–––, 2010, “Defining Life: the Virus
Viewpoint,”
Origins of Life and Evolution of
Biospheres
, 40(2): 151–160.
Frank, Adam and Woody T. Sullivan III, 2016, “A new
empirical constraint on the prevalence of technological species in the
universe,”
Astrobiology
, 16(5): 359–362.
Goodpaster, Kenneth E., 1978, “On being morally
considerable,”
The Journal of Philosophy
, 75(6):
308–325.
Gopnik, Alison, and Andrew N. Meltzoff, 1997,
Words, Thoughts,
and Theories
, Cambridge, Mass.: MIT Press.
Groff-Vindman, Cindy S., Trump, Benjamin D., Cummings, Christopher
L., Smith, Madison, Titus, Alexander J., Oye, Ken, Prado, Valentina,
Turmus, Eyup, & Linkov, Igor, 2025, “The Convergence of AI
and Synthetic Biology: the Looming Deluge,”
NPJ Biomedical
Innovations
, 2(1): 20.
Gromski, Piotr S., Granda, Jaroslaw M. and Cronin, Leroy, 2020,
“Universal chemical synthesis and discovery with ‘the
chemputer,’”
Trends in Chemistry
, 2(1):
4–12.
Gupta, Anil and Mackereth, Stephen“Definitions,”,
The Stanford Encyclopedia of Philosophy
(Fall 2023 Edition),
E.N. Zalta (ed.), URL =
>.
Haldane, John Burdon Sanderson, 1929, “The Origin of
Life,”
Rationalist Annual
, 148: 3–10.
Hampton, James A., 1979, “Polymorphous concepts in semantic
memory,”
Journal of Verbal Learning and Verbal
Behavior
, 18: 441–461.
–––, 2006, “Concepts as prototypes,”
in B.H. Ross (ed.),
The Psychology of Learning and Motivation:
Advances in Research and Theory
(Volume 46), New York: Academic
Press, pp. 79–113.
Haqq-Misra, Jacob, 2024, “Technosignatures and
Astrobiology,” in
Oxford Research Encyclopedia of Planetary
Science
, Oxford: Oxford University Press.
doi:10.1093/acrefore/9780190647926.013.275
He, Christine, Isaac Gállego, Brandon Laughlin, Martha A.
Grover, and Nicholas V. Hud, 2017, “A Viscous Solvent Enables
Information Transfer from Gene-Length Nucleic Acids in a Model
Prebiotic Replication Cycle,”
Nature Chemistry
, 9(4):
318–324.
Hölldobler, Bert and Wilson, Edward O., 2009,
The
Superorganism: the Beauty, Elegance, and Strangeness of Insect
Societies
, New York: W.W. Norton & Company.
Hutchison, Clyde A., Ray-Yuan Chuang, Vladimir N. Noskov, Nacyra
Assad-Garcia, Thomas J. Deerinck, Mark H. Ellisman, John Gill, Krishna
Kannan, Bogumil J. Karas, Li Ma, James F. Pelletier, Zhi-Qing Qi, R.
Alexander Richter, Elizabeth A. Strychalski, Lijie Sun, Yo Suzuki,
Billyana Tsvetanova, Kim S. Wise, Hamilton O. Smith, John I. Glass,
Chuck Merryman, Daniel G. Gibson, J. Craig Venter, 2016, “Design
and synthesis of a minimal bacterial genome,”
Science
351(6280). doi:10.1126/science.aad6253
Jabr, Ferris, 2013, “Why Life Does Not Really Exist,”
Scientific American
, published 02 December 2013
Jabr 2013 available online
Joyce, Gerard F., 1994, forward, in D. W. Deamer, G. R.
Fleischaker (eds.),
Origins of Life: the Central Concepts
Boston: Jones & Bartlett, pp. xi–xii.
Kelley, David, 2024, “Philosophy Moves,”
Australasian Journal of Philosophy
, 102(3):
537–550.
Kendal, E., 2025,
Science Fiction and the Ethics of Artificial
Wombs: Reproductive Futures
, London: Bloomsbury Publishing.
Komatsu, Lloyd K., 1992, “Recent Views of Conceptual
Structure,”
Psychological Bulletin
, 112:
500–526.
Koonin, Eugene V., 2000, “How Many Genes can Make a Cell:
the Minimal-Gene-Set Concept,”
Annual Review of Genomics and
Human Genetics
, 1(1): 99–116.
–––, 2009, “On the Origin of Cells and
Viruses: Primordial Virus World Scenario,”
Annals of the New
York Academy of Sciences
, 1178(1): 47–64.
–––, 2011,
The Logic of Chance: the Nature
and Origin of Biological Evolution
, New Jersey: FT press.
Koonin, Eugene V., Tatiana G. Senkevich, and Valerian V. Dolja,
2006, “The Ancient Virus World and Evolution of cells,”
Biology Direct
, 1(1): 1–27.
Langton, Christopher G., 1989, “Artificial life,” in
C.G. Langton (ed.),
Artificial life
(Santa Fe Institute
Studies in the Sciences of Complexity: Proceedings Volume IV), Redwood
City, Calif.: Addison-Wesley, pp. 1–47.
–––, 1995,
Artificial Life: An
Overview
, Cambridge, Mass.: MIT Press.
Lenton, Timothy M., Stuart J. Daines, James G. Dyke, Arwen E.
Nicholson, David M. Wilkinson, and Hywel T.P. Williams, 2018,
“Selection for Gaia Across Multiple Scales,”
Trends in
Ecology & Evolution
, 33(8): 633–645.
Leopold, Aldo, 1949,
A Sand County Almanac
, New York:
Oxford University Press.
Levin, Gilbert V. and Patricia Ann Straat, 1976, “Viking
labeled release biology experiment: interim results,”
Science
, 194(4271): 1322–1329.
Li, Jiali, Shawn Browning, Sukhvir P. Mahal, Anja M. Oelschlegel,
and Charles Weissmann, 2010, “Darwinian evolution of prions in
cell culture,”
Science
, 327(5967): 869–872.
Lincoln, Tracey A. and Joyce, Gerald F., 2009,
“Self-sustained replication of an RNA enzyme,”
Science
, 323(5918): 1229–1232.
Lineweaver, Charley H., 2020, “What Do the DPANN Archaea and
the CPR Bacteria Tell Us about the Last Universal Common
Ancestors?” in J. Seckbach and H. Stan-Lotter (eds.),
Extremophiles as Astrobiological Models
, London: Wiley,
pp.359–367.
Loeb, Abraham and Dan Maoz, 2013, “Detecting Biomarkers in
Habitable-Zone Earths Transiting White Dwarfs,”
Monthly
Notices of the Royal Astronomical Society: Letters
, 432(1):
L11–L15.
Lovelock, James E., 2000,
Gaia: A New Look at Life on
Earth
, Oxford: Oxford University Press.
Machery, Edouard, 2009,
Doing without Concepts
, Oxford:
Oxford University Press.
–––, 2011, “Why I Stopped Worrying About
the Definition of Life… and Why you Should as Well,”
Synthese
, 185(1): 145–164. doi:
10.1007/s11229-011-9880-1
Madhusudhan, Nikku, Constantinou, Savvas, Holmberg, Måns,
Sarkar, Subhajit, Piette, Anjali A., & Moses, Julianne I., 2025,
“New Constraints on DMS and DMDS in the Atmosphere of
K2–18 b from JWST MIRI,”
The Astrophysical Journal
Letters
, 983(2): L40.
Maienschein, Jane, 2014,
Embryos Under the Microscope
Cambridge, Mass.: Harvard University Press.
Malaterre, Christophe and Jean-François Chartier, 2019,
“Beyond Categorical Definitions of Life: a Data-Driven Approach
to Assessing Lifeness,”
Synthese
, 198:
4543–4572.
Malaterre, Christoph, Jeancolas, Cyrille, & Nghe, Philippe,
2022, “The Origin of Life: What is the Question?”
Astrobiology
, 22(7): 851–862.
Malaterre, Christoph, Ten Kate, Inge Loes, Baqué, Mickael,
Debaille, Vinciane, Grenfell, John Lee, Javaux, Emmanuelle J.,
Khawaja, N., Klenner, F. Lara, Y.J, McMahon, Sean, and Moore, K.,
2023, “Is there Such a Thing as a Biosignature?”
Astrobiology
, 23(11): 1213–1227.
Mariscal, Carlos, Ana Barahona, Nathanael Aubert-Kato, Arsev Umur
Aydinoglu, Stuart Bartlett, María Luz Cárdenas, Kuhan
Chandru, Carol E. Cleland, Benjamin T. Cocanougher, Nathaniel Comfort,
and Athel Cornish-Bowden, 2019, “Hidden concepts in the history
and philosophy of origins-of-life studies: A workshop report,”
Origins of Life and Evolution of Biospheres
, 49(3):
111–145.
Mariscal, Carlos and W. Ford Doolittle, 2015, “Eukaryotes
first: how could that be?”
Philosophical Transactions of the
Royal Society B
(Biological Sciences), 370(1678): 20140322.
–––, 2020, “Life and Life Only: a Radical
Alternative to Life Definitionism,”
Synthese
, 197(7):
2975–2989.
Mathis, Cole, Tanmoy Bhattacharya, and Sara Imari Walker, 2017,
“The Emergence of Life as a First-Order Phase Transition,”
Astrobiology
, 17(3): 266–276.
McKay, David S., Everett K. Gibson, Kathie L. Thomas-Keprta,
Hojatollah Vali, Christopher S. Romanek, Simon J. Clemett, Xavier D.F.
Chillier, Claude R. Maechling, and Richard N. Zare, 1996,
“Search for past life on Mars: possible relic biogenic activity
in Martian meteorite ALH84001,”
Science
, 273(5277):
924–930.
Medin, Douglas L. and Marguerite M. Schaffer, 1978, “Context
Theory of Classification Learning,”
Psychological
Review
, 85(3): 207–238.
Miller, Stanley L., 1953, “A Production of Amino Acids Under
Possible Primitive Earth Conditions,”
Science
117(3046): 528–529.
Mix, Lucas John, 2018,
Life Concepts from Aristotle to Darwin:
On Vegetable Souls
, New York: Springer.
–––, 2020, “Three Lives and
Astrobiology,” in K.C. Smith and C. Mariscal (eds.)
Social
and Conceptual Issues in Astrobiology
, Oxford: Oxford University
Press, pp. 57–78.
Moody, Edmund R.R., Álvarez-Carretero, Sandra,
Mahendrarajah, Tara A., Clark, James W., Betts, Holly C., Dombrowski,
Nina, Szántho, Lénárd L., Boyle, R.A., Daines,
S., Chen, X., and Lane, Nick, 2024, “The Nature of the Last
Universal Common Ancestor and its Impact on the Early Earth
System,”
Nature Ecology & Evolution
, 8(9):
1654–1666.
More, Max and Natasha Vita-More, (eds.), 2013,
The
Transhumanist Reader: Classical and Contemporary Essays on the
Science, Technology, and Philosophy of the Human Future
, London:
John Wiley & Sons.
Moreira, David and Purificación López-García,
2009, “Ten reasons to exclude viruses from the tree of
life,”
Nature Reviews Microbiology
, 7(4):
306–311.
Murphy, Gregory L., and Douglas L. Medin, 1985, “The Role of
Theories in Conceptual Coherence,”
Psychological
Review
, 92(3): 289–316.
Nicholson, Daniel J. and Dupré, John, 2018,
Everything
Flows: Towards a Processual Philosophy of Biology
, Oxford: Oxford
University Press.
Noonan John T. Jr., 1967, “Abortion and the Catholic Church:
A Summary History,”
National Law Forum
, 12:
85–131.
Nosofsky, Robert M., 1986, “Attention, similarity, and the
identification categorization relationship,”
Journal of
Experimental Psychology: Learning, Memory, and Cognition
, 115(1):
39–57.
Oparin, Aleksandr Ivanovich, 1961,
Life: Its Nature, Origin
and Development
, New York: Academic Press.
–––, 1936 [2010], “The Nature of
Life,” in M.A. Bedau and C.E. Cleland,
The Nature of Life:
Classical and Contemporary Perspectives from Philosophy and
Science
, New York: Cambridge University Press; this translation
by Ann Synge was first published in
A. I. Oparin, Life: Its
nature, origin, and development
, New York: Academic Press, 1964,
pp. 1–37.
Pace, Norman R., 2001, “The Universal Nature of
Biochemistry,”
Proceedings of the National Academy of
Sciences
, 98(3): 805–808.
Pályi, Gyula, Claudia Zucchi, and Luciano Caglioti, (eds.),
2002,
Fundamentals of Life
, Paris: Elsevier.
Parke, Emily C., 2020, “Dimensions of Life
Definitions,” in K.C. Smith & C. Mariscal (eds.),
Social
and Conceptual Issues in Astrobiology
, Oxford: Oxford University
Press, pp. 79–90.
Pearce, Ben K.D., Andrew S. Tupper, Ralph E. Pudritz, and Paul G.
Higgs, 2018, “Constraining the time interval for the origin of
life on Earth,”
Astrobiology
, 18(3):
343–364.
Peplow, Mark, 2005, “Formaldehyde claim inflames Martian
debate,”
Nature
, published online 25 February 2005.
doi:10.1038/news050221-15
Peretó, Juli, Jeffrey L. Bada, and Antonio Lazcano, 2009,
“Charles Darwin and the origin of life,”
Origins of
Life and Evolution of Biospheres
, 39(5): 395–406.
Plato,
Phaedrus
, in
Plato: Complete Works
, J.
Cooper and D. Hutchinson (eds.), Indianapolis: Hackett, 1997.
–––,
The Republic
, in
Plato:
Complete Works
, J. Cooper and D. Hutchinson (eds.), Indianapolis:
Hackett, 1997.
–––,
Timeaus
, in
Plato: Complete
Works
, J. Cooper and D. Hutchinson (eds.), Indianapolis: Hackett,
1997.
Popa, Radu, 2004,
Between Necessity and Probability: Searching
for the Definition and Origin of Life
, New York: Springer.
Postgate, John, 2000, “The Twilight Zones of
Microbiology,”
Microbiology Today
, 27:
162–163.
Raman, Ritu, 2024, “Biofabrication of Living
Actuators,”
Annual Review of Biomedical Engineering
26(1): 223–245.
Ramsey, William, “Eliminative Materialism,”
The
Stanford Encyclopedia of Philosophy
(Summer 2020 Edition), Edward
N. Zalta (ed.),URL =
>.
Ray, Thomas S., 1993, “An evolutionary approach to synthetic
biology: Zen and the art of creating life,”
Artificial
Life
, 1(1/2): 179–209.
Rolston III, H., 2017, “Is there an Ecological Ethic?”
in M. Wachs (ed.),
Ethics in Planning
, London: Routledge, pp.
299–317.
Rosch, Eleanor, and Mervis, C. B., 1975, “Family
resemblance: Studies in the internal structure of categories,”
Cognitive Psychology
, 7: 573–605.
Rosch, Eleanor, 1978, “Principles of Categorization,”
in E. Rosch and B.B. Lloyd (eds.),
Cognition and
Categorization
, Hillsdale, NJ: Erlbaum, pp. 27–48.
Riskin, Jessica, 2016,
The Restless Clock: A History of the
Centuries-Long Argument Over what Makes Living Things Tick
Chicago: University of Chicago Press.
Sagan, Carl, 1970, “Life,”
Encyclopedia
Britannica
, Chicago: Encyclopædia Britannica Incorporated,
pp. 1083–1083A.
Scharf, Caleb, Nathaniel Virgo, H. James Cleaves III, Masashi
Aono, Nathanael Aubert-Kato, Arsev Aydinoglu, Ana Barahona, Laura M.
Barge, Steven A. Benner, Martin Biehl, Ramon Brasser, Christopher J.
Butch, Kuhan Chandru, Leroy Cronin, Sabastian Danielache, Jakob
Fischer, John Hernlund, Piet Hut, Takashi Ikegami, Jun Kimura, Kensei
Kobayashi, Carlos Mariscal, Shawn McGlynn, Brice Menard, Norman
Packard, Robert Pascal, Juli Pereto, Sudha Rajamani, Lana Sinapayen,
Eric Smith, Christopher Switzer, Ken Takai, Feng Tian, Yuichiro Ueno,
Mary Voytek, Olaf Witkowski, and Hikaru Yabuta, 2015, “A
Strategy for Origins of Life Research,”
Astrobiology
15(12): 1031–1042.
Schopf, J. William, 1993, “Microfossils of the Early Archean
Apex chert: new evidence of the antiquity of life,”
Science
, 260(5108): 640–646.
Schopf, J. William, Kouki Kitajima, Michael J. Spicuzza, Anatoliy
B. Kudryavtsev, and John W. Valley, 2018, “SIMS analyses of the
oldest known assemblage of microfossils document their
taxon-correlated carbon isotope compositions,”
Proceedings
of the National Academy of Sciences
, 115(1): 53–58.
Schulze-Makuch, Dirk, Huade Guan, Louis N. Irwin, E Vega, 2002,
“Redefining life: An ecological, thermodynamic, and
bioinformatic approach,” in G. Pályi, C. Zucchi, L.
Caglioti (eds.),
Fundamentals of Life
, New York: Elsevier,
pp. 169–179.
Seager, Sara, Welbanks, Luis, Ellerbroek, Lucas, Bains, William,
& Petkowski, Janusz J., 2025, “Prospects for Detecting Signs
of Life on Exoplanets in the JWST Era,”
Proceedings of the
National Academy of Sciences
, 122(39): e2416188122.
Slater, Matthew H., 2015, “Natural Kindness,”
The
British Journal for the Philosophy of Science
, 66(2):
375–411.
Smith, J. David, 2002, “Exemplar Theory’s Predicted
Typicality Gradient can be Tested and Disconfirmed,”
Psychological Science
, 13(5): 437–442.
Smith, Harrison B., & Mathis, Cole, 2023, “Life
Detection in a Universe of False Positives: Can the Fatal Flaws of
Exoplanet Biosignatures be Overcome Absent a Theory of Life?”
BioEssays
, 45(12): 2300050.
Smith, Kelly C., 2020, “METI or REGRETTI,” in K.C.
Smith and C. Mariscal (eds.),
Social and Conceptual Issues in
Astrobiology
, Oxford: Oxford University Press, pp.
209–235.
Sober, Elliott, 1991, “Learning from Functionalism:
prospects for strong artificial life,” in C.G. Langton, C.
Taylor, J.D. Farmer, and S. Rasmussen (eds.),
Artificial life
II (Santa Fe Institute Studies in the Sciences of Complexity:
Proceedings Volume X), Redwood City, CA: Addison-Wesley, pp.
749–765.
Sterelny, Kim, 1993, “Categories, Categorisation and
Development: Introspective Knowledge is No Threat to
Functionalism,”
Behavioral and Brain Sciences
, 16(1):
81–83.
Stewart, Potter, 1964, “Jacobellis vs. Ohio,” 378 U.S.
184, 197.
Szostak, Jack W., 2012, “Attempts to Define Life Do Not Help
to Understand the Origin of Life.”
Journal of Biomolecular
Structure & Dynamics
, 29(4): 599–600. doi:
10.1080/073911012010524998
Trifonov, Edward N., 2011, “Vocabulary of definitions of
life suggests a definition.”
Journal of Biomolecular
Structure and Dynamics
, 29(2): 259–266
Trombley, Carolyn A. and Karl Cottenie, 2019, “Quantifying
the scientific cost of ambiguous terminology in community
ecology,”
Philosophical Topics
, 47(1):
203–218.
Tsokolov, Serhiy A., 2009, “Why Is the Definition of Life So
Elusive? Epistemological Considerations,”
Astrobiology
9(4): 401–412. doi:10.1089/ast.2007.0201
Wächtershäuser, Günter, 1988, “Before Enzymes
and templates: Theory of Surface Metabolism,”
Microbiological Reviews
, 52(4): 452–484.
Weber, Bruce H., 2007, “Emergence of life,”
Zygon
, 42: 837–856.
–––, 2010, “What is Life? Defining Life in
the Context of Emergent Complexity,”
Origins of Life and
Evolution of Biospheres
, 40: 221–229.
Webster, Christopher R., Paul R. Mahaffy, Sushil K. Atreya, John
E. Moores, Gregory J. Flesch, Charles Malespin, Christopher P. McKay,
German Martinez, G., Christina L. Smith, Javier Martin-Torres, Javier
Gomez-Elvira, Maria-Paz Zorzano, Michael H. Wong, Melissa G. Trainer,
Andrew Steele, Doug Archer Jr., Brad Sutter, Patrice J. Coll, Caroline
Freissinet, Pierre-Yves Meslin, Raina V. Gough, Christopher H. House,
Alexander Pavlov, Jennifer L. Eigenbrode, Daniel P. Glavin, John C.
Pearson, Didier Keymeulen, Lance E. Christensen, Susanne P. Schwenzer,
Rafael Navarro-Gonzalez, Jorge Pla-García, Scot C.R. Rafkin,
Álvaro Vicente-Retortillo, Henrik Kahanpää, Daniel
Viudez-Moreiras, Michael D. Smith, Ari-Matti Harri, Maria Genzer,
Donald M. Hassler, Mark Lemmon, Joy Crisp, Stanley P. Sander, Richard
W. Zurek, Ashwin R. Vasavada, 2018, “Background Levels of
Methane in Mars’ Atmosphere Show Strong Seasonal
Variations,”
Science
, 360(6393): 1093–1096.
Wells, Herbert George, 1898,
The War of the Worlds
London: William Heinemann.
White, Lauren M., Everett K. Gibson, Kathie L. Thomas-Keprta,
Simon J. Clemett, and David S. McKay, 2014, “Putative Indigenous
Carbon-Bearing Alteration Features in Martian Meteorite Yamato
000593,”
Astrobiology
, 14(2): 170–181.
Woese, Carl R. and George E. Fox, 1977, “The Concept of
Cellular Evolution,”
Journal of Molecular Evolution
10(1): 1–6.
Woese, Carl R., 1998, “The Universal Ancestor,”
Proceedings of the National Academy of Sciences
, 95(12):
6854–6859.
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Darwin, C.,
Letter to J. D. Hooker, 01 February 1871
Letter no. 7471, Darwin Correspondence Project.
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