## 1. Definition(s)



Few things in biology have been more extensively 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: disagreements about
how abstract or specific definitions should be, different commitments
as to what ought to be included in a definition, and even disagreement
about the nature of definitions themselves. This section covers the
nature and role of definitions. Each of these has been used in approaching
the question of life.


Historically, when philosophers and scientists define a concept,
the aim is to provide necessary and sufficient conditions. These
*theoretical* definitions (also
called *real*, *ideal* or *philosophical*
definitions) are often impractical or fragile as they can be
challenged with a single imagined counterexample. A classic case is
the definition of "bachelors" as "unmarried
males." It is trivial to find examples that fit this definition
without intuitively being bachelors: male dogs, baby boys, widowers,
etc. Similarly, for any definition of life, one can either show living
cases that are left out of the definition or non-living cases that are
included by it. 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. As we'll see shortly, perhaps 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 typically quite frustrated by the
back-and-forth that results from theoretical definitions. To that end,
some favor *operational* (or *working*) definitions,
ones that 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 2021). Operational definitions
tend to be philosophically shallow. For example, NASA's
operational definition of life as "a self-sustaining chemical
system capable of Darwinian evolution" (Joyce 1994) might
include viruses while excluding mules. The lack of depth of an
operational definition can frustrate theoretically minded people,
including other scientists.


There are several other conceptions of definition, as well. The
*nominal* (also *lexicographer* or *dictionary*)
definition is determined by analyzing usage. It will not work for
cutting edge or controversial issues because such definitions follow
the slow process of cultural acceptance rather than provide guidance
to researchers at the forefront of these debates. Scholars are more
likely to quote a dictionary definition than be illuminated by
one.


There are also *demonstrative* or *ostensive*
definitions, which are concepts we can convey by mere shared
observations: "that is red" while pointing at a red
object, for instance. Potter Stewart famously defined pornography in
this manner by saying "I know it when I see it" (Stewart
1964). Knowledge by ostension may reflect epistemic access to a
natural kind, although this may feel indistinguishable from an
internalized cultural category. There is huge variation among what
scientists consider 'life,' even among objects on Earth,
like viruses and prions, which suggests this kind of definition is not
viable for this target.


Then there are *stipulative* definitions, which are terms introduced
and defined by fiat. A circle in Euclidean geometry can be defined as a
round plane figure whose boundary consists 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
perhaps only works within accepted axioms or theories. Life could be
stipulated as "carbon-based reproducing entities," which
would rule out silicon-based life by fiat. Such a definition merely
pushes the debate to the scenarios in which the stipulated definition
goes against intuitive notions.


The 20th century saw some steps away from definitions toward
alternative views of concepts,
notably *prototypes*, *exemplars*, and
*theories* (Machery 2009). *Prototype* concepts are
abstract features shared by most, but not all members of a category
(Rosch & Mervis 1975; Rosch 1978; Hampton 1979, 2006; Smith
2002). The definitions of life in biology textbooks might be
charitably understood as prototype concepts. 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 paradigmatic cases, with the
former being an imagined ideal and the latter being a real instance
(Komatsu 1992). 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, the
requirements of theory concepts are somewhat more
nebulous. *Theory* concepts are modeled on scientific theories
and thus reflect their diversity (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. Marginal cases of life, such as viruses, prions, or
protocells, might be included in some theories of life but not
others.


In sum, there are many potential approaches to definitions, each
with different benefits, drawbacks, and standards of success. Much more
could be said about these and other possible approaches, but this will
suffice for our purposes. Some of the disagreements about defining life
dissolve upon clarifying which type of definition or concept is being
used. Many of the explicit attempts to define life have focused on
either operational and philosophical definitions, while often not
acknowledging or misunderstanding the distinction between these. One
will note these two definitions are at cross-purposes -
operational definitions can be quick and dirty, but philosophical
definitions seek to give necessary and sufficient conditions. Less work
has been done on life as a non-definitional concept, although that is
perhaps changing.


### 1.1 Definitions of Life from Antiquity to Darwin




This subsection briefly 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 widely across historians, philosophers,
and scientists, so some skepticism about any individual author's
approach to the topic is warranted.


We begin with the Greeks. In several dialogues, notably 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 were additionally capable of
sensation and locomotion, and humans also had rational souls.
Plato's subsequent influence in Christian theology may be
apparent in spirit if not in detail. In Christian theology, human life
was not only rational, but also involved an eternal, spiritual soul and
an internal, conscious life.


Plato's student, Aristotle, had a different notion in which
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). For
Aristotle, the capacity to resist internal and external perturbations
was the essential distinction between living beings and non-living
objects. Other features were accidental. This quest for demarcating the
essential from the accidental for life has persisted to this day in
searches for theoretical definitions of life as well as in attacks
against those not interested in such definitions.


Centuries later, Descartes drew a sharper distinction between animal
life and rational life than between inanimate objects and animal life.
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). As such,
Descartes' category of life neither mapped onto Greek conceptions
nor current conceptual frameworks. The mechanistic view developed by
Descartes and his followers is often thought to be continuous with
current scientific thinking, but this is perhaps anachronistic, as much
of the theoretical underpinning separating animal life and rational
life is no longer accepted.


 The responses to Descartes came to be grouped under the
heading 'vitalism.' Vitalism, which spanned three
centuries, was a heterogenous philosophical position unified by
adherents' doubt of a fully mechanistic view of life. Vitalists
had ontologies of defining features of life as varied as immaterial
causes, particular arrangements of matter, a special life fluid, a
particular end goal, or even mental forces. A whiggish history of
biology will declare the death of vitalism with Friedrich
Wohler's synthesis of urea from ammonium cyanate. The
suggestion is that if biological chemicals can be produced from mere
chemistry, then biology is also mere chemistry. Although this was an
important step, many chemists already had accepted a mechanistic world
view, and many other 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 largely saw the mechanist/vitalist divide
dissipate. Despite the difficulties described above about definitions,
hundreds of scientists, philosophers, and others have tried their hand
at defining life. Much of the interest is motivated by new science and
new technologies - including artificial life, synthetic biology,
origins of life, and astrobiology - which complicate the issue
by violating some of the traditional groupings of properties
associated with life. There are numerous books, articles, and
workshops on the nature of life (Palyi 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, primarily 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 purposefully 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. For example, Popa 2004 considers definitions ranging
from Oparin 1961's "Any system capable of replication and
mutation is alive" to Schulze-Makuch et al. 2002:



> 
> We propose to define living systems as those that are: (1)
> composed of bounded micro-environments in thermodynamic equilibrium
> with their surroundings; (2) capable of transforming energy to maintain
> their low-entropy states; and (3) able to replicate structurally
> distinct copies of themselves from an instructional code perpetuated
> indefinitely through time despite the demise of the individual carrier
> through which it is transmitted. 


Categorizing definitions such as these
necessarily requires some choices and reasonable people can disagree
about whether each belongs in one or more categories.


The takeaway from current understandings of the definition of life
is that there is no consensus forthcoming in the near future. 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 scientists would include prions, viruses, and
entities only hypothesized to exist in the origin of life, while others
would completely reject them. Some might accept digital organisms as
alive, others would deny this approach. Conceptual equivocation could
have significant costs for research. One field recently quantified this
cost, suggesting it is more than a merely theoretical concern (Trombley
and Cottenie 2019). Given the diversity described above, one may be
tempted 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



Nearly everybody agrees there is a distinction between life and
non-life, typically understood as a difference in kind rather than one
of degree. Furthermore, most people involved accept that life is some
sort of natural kind, rather than a human psychological concept. That
said, a common theme in recent philosophical work 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 have disagreed as to the ultimate source of the lack of
consensus, citing unstated assumptions in either the definer's
approach or the question itself. Note that many scientists are less
likely to be skeptical of the goal of defining life, though also more
resistant to engaging in the philosophical debate.


One skeptical view has arisen from the observation that theoretical
definitions of life presume a theory of life (Cleland and Chyba 2002,
Benner 2010, Cleland 2019). Although it is not obvious that the authors
allude 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 H2O, while the other
'waters' were HNO3 + 3 HCl and
HNO3, 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. Relatedly, most believe life is some kind
of entity rather than some kind of relation or process (but see
Nicholson and Dupre 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 famously worried about biochemical definitions
because they were prone to 'Earth Chauvinism' for
privileging our own biochemistry (1970). Other authors take our
biochemistry to be independently justified as universal (Pace 2001,
Benner et al. 2004, but see Bains 2004). 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 denied the distinction between life & non-life entirely (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
truly exist. We're all eliminativists about something, perhaps
ghosts or fairies. Linguistic/conceptual eliminativists, on the other
hand are merely 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
actually exists in the world while still accepting it as a useful fiction. Conversely, one may 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,
especially 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. Some of the definitions described above are derived from, or
necessary for, specific scientific and societal purposes. This section
focuses on artificial and synthetic life.


In principle, most contemporary scientists and philosophers believe
life can be created, but there is broad disagreement as to what needs
to be recreated for something to be life. In functional approaches,
mere formal organization sufficiently similar to organisms may be
enough. Complexly configured robots ("hardware") or computer programs ("software") might qualify. This view is known as Strong Artificial Life (A-Life for short) and has received much of the same pushback as the Strong Artificial Intelligence
approach before it (Sober 1991, Boden 1999, Brooks 2001). Those who
reject the Strong A-Life view believe that functional approaches miss
some of the essential features of biology for either epistemic or
ontological reasons. Epistemic objections might be consistent with the
possibility of Strong A-Life, but doubt that we have the knowledge to
recreate the relevant biological functions in a digital framework.
Conversely, most of the objections to Strong A-Life have been
ontological, resting on the view that representations cannot be
equivalent to that which they represent and that perhaps life requires chemical embodiment, ruling Strong A-Life 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, effectively 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 quickly and efficiently.


Another approach worth highlighting is that of synthetic life
("wetware"). Less conceptually troubled, 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), a chemical evolution (Gromski et al.
2020), and other projects. Not all synthetic biology is in the business
of investigating life as it could be, as not all computer programming
is A-Life. Nevertheless, the tools developed by both can be
illuminating. By exploring possibilities, scientists can discover
previously hidden relationships, revealing which aspects of life are
more or less plausible than expected.


## 4. Origin(s) of Life




Inextricable from the question of life's nature is the
question of its origin. Ancient and modern thinkers accepted that life
often arose spontaneously from non-life. Two centuries of experiments
eventually overturned this widely accepted view, culminating in Louis
Pasteur's swan-neck bottle experiments. Since then, the puzzle of
Life's origin has been one of the biggest and most important in
all of science.


Darwin was famously silent about the problem, although 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 Pereto et al. 2009).
Subsequent work on the subject was sparse until the 1920s when
Alexander Oparin and J.B.S. Haldane independently proposed hypotheses
for life's origin in then plausible early Earth conditions
(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 distinctive evidence of microbial fossils, including
distinctive shapes that correlate to the sizes and shapes of current
prokaryotes, as well as carbon-ratios distinctive to life as we know it
(Schopf 1993, Schopf et al. 2017). Many analyses have pushed our
confidence in life's earlier origin significantly further back,
suggesting that basically 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. Several of these are centered around the explanandum in
question and epistemological limits to our knowledge. Researchers
differ, for example, as to whether the purpose of origins-of-life
research is to discover how life *could have* originated or how it *did*
originate (Scharf et al. 2015, Mariscal et al. 2019). Some steps in the
process could have been chancy, others could have been deterministic
but highly 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 cleverly
attempted to ease the problem by relaxing assumptions: perhaps the
environment provided our first boundaries (Koonin 2009), or perhaps it
provided porto-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 (Wachtershauser 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, primarily 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, presumably in Earthly pre-biotic conditions, led to the
existence of LUCA, an important constraint upon theorizing about the
origin of life. It is widely expected that LUCA was merely 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 it was a heterotroph or autotroph; where it lived; and
when it lived.


The gap between Top-Down and Bottom-Up approaches is huge: 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: the chemical byproducts of life and the technological signals
of *intelligent* life. The social interactions of alien populations might
be interesting, but they are hard to study as of yet. Thus, we search
for biosignatures that might uniquely identify life from a great
distance. We'll take each in turn.


Biosignatures, as the name implies, are purported to be markers of
life. Chemical biosignatures are compounds either rarely or never
produced without the assistance of life on Earth. Finding biosignatures
thus implies a material conception of life, likely in the form of
biochemistry, metabolism, or thermodynamics. There have been many
attempts to detect biosignatures, primarily 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.


More practically, 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). Venus has also been a source
of attention, with phosphine gas detected in the clouds above Venus
(Greaves et al. 2020). A controversial finding, it nevertheless caught
the scientific imagination. Future scientific research is expected to
accelerate this method of observation, especially with new data gathered by the new
James Webb Space Telescope (JWST). With its equipment, JWST is able to image exoplanets at resolutions allowing the detection of gas biomarkers in the atmospheres of exoplanets (Loeb & Maoz 2013).


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 relatively 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. The search for
extraterrestrial intelligence (SETI) has been ongoing for centuries
(Dick 1982). Fierce debates between those that took Earth to be
unique and those that took it to be one of a plurality of worlds
persisted for millennia (and still do, to some extent). Advocates of
the plurality of worlds view searched their telescopes for evidence. A
famous instance is Percival Lowell's drawings of Martian canals
in the 1890s. Influenced by the mid-1800s observations of what appeared
to be channels criss-crossing 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.


Partially driven by the science fiction following Lowell's
drawings, the early 20th century saw increased interest in detecting
radio signals from Outer Space. This interest accelerated after the
launch of Sputnik in 1957 and was more systematically and formally
approached starting in the late 1970s. 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.


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 largely 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 grow more concerned about philosophical questions when
scientific limitations or conceptual choices are made apparent to them.
Those scientists who study deep time, deep space, abstract issues, or
questions of ethics are often keenly aware of the philosophical choices
that influence their research from identification of research question
to interpretation of the data. This section briefly goes through
other scientific contexts in which how life is defined is relevant,
which address scales well below and well 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
Lopez Garcia 2009) nor 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," including transposable elements, viroids,
unculturable (but putatively existing) microbes, organisms in
vegetative states, and prions (Postgate 1999). The problems facing each
of these are similar: several of them can evolve by natural selection,
are biochemically complex, but lack other 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 origin as a
misfold of a functional protein.


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 (Holldobler & 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. This is
perhaps a general feature of life: 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 2019). 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 biology. Often,
the focus is not on the concept of life or life in general, but on the
status of individual living entities. Typically, the focus is on the
beginning and end stages of individual lives, which raises legal,
religious, and moral questions. The start of an individual life
has been the source of contraception and abortion discussions 
(Noonan 1967, Dellapenna 1978). Unfortunately, developmental biology
does not provide an uncontroversial starting point for when
'life' begins (Maienschein 2014). 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).


Any discussion of defining life in these contexts should begin by
distinguishing between life and other phenomena that are often
conflated with it in public discourse, such as sentience, personhood,
and moral considerability. It's unclear how much ontological,
epistemic, or moral weight the category of life has independently of
other properties. Attributing moral worth to non-living entities is
still a minority position in environmental or comparative philosophy
(but see Leopold 1949, Basl 2019). Thus, a starting position might be
that life is a prerequisite for moral considerability. Nonetheless,
most humans don't mind killing bacteria for the sake of
cleanliness and many people eat or wear the flesh of animals. So, in
many discussions, life is only valuable when it is the vehicle for
other equally nebulous properties like sentience, personhood, or
immaterial souls.


If any living entities have a distinct moral or ontological status,
most philosophers would 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. Considered
opinions differ as to when this occurs and in virtue of what, be it
mere possibility of self-sustained life, sentience, or other features.
There is still a rampant pro-life/pro-choice split in cultural
politics, which is somewhat lessened in European countries (Corbella
2020). 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.


The public 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. 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. And although it is beyond the scope of this article
to adjudicate these debates, advocates ought to be aware of the deep
vagueness in biology and disagreement within philosophy 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.


## 8. Conclusion




Although the conceptual terrain of life concepts is well covered,
there is no accepted view as yet. This is unlikely to change given the
disciplinary backgrounds, explanatory values, and theoretical
commitments of the stakeholders involved. A wide range of practices
rely on competing conceptions of life: including artificial life,
origins-of-life research, the search for life, and other projects
described above.


Future scientific discoveries or inventions may break this impasse,
as they have in other cases of theoretical gridlock. The development of
atomic theory, discussed in section 2, created new categorical
divisions that scientists accepted as more real than the categories of
the ancients or alchemists. With this conceptual fragmentation, old
categories were discarded and new ones accepted. 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, eventually winning over the majority of the
scientific and philosophical community (e.g. Weber 2007, 2010).


Conversely, a simple decision might be made based on shared values
or explanatory goals. The example of death may be illustrative. After
decades of debate, a new decision, not a discovery, was made.
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, perhaps life will be accepted as a polysemous concept with
each definitional cluster applying to a subset of the whole:
biochemical life, evolutionary life, metabolic life, etc. Researchers
may rely on context, accept some miscommunication, or simply 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. Initially,
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. From 1992 until 2006, many objects similar to
Pluto were discovered until astronomers decided that the term planet
actually covered at least two distinct, but scientifically 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 before a discovery forces a scientific reckoning or a
decision obviates the need. For now, the debate continues.