It is the mark of an educated man to look for precision in each class of things just so far as the nature of the subject admits; it is evidently equally foolish to accept probable reasoning from a mathematician and to demand from a rhetorician scientific proofs.—Aristotle, Nichomachean Ethics, Book I, 1094b.24.
Abstract
Whatever we take “life” to mean, it must involve an attempt to describe the objective reality beyond scientists’ biases. Traditionally, this is thought to involve comparing our scientific categories to “natural kinds.” But this approach has been tainted with an implicit metaphysics, inherited from Aristotle, that does not fit biological reality. In particular, we must accept that biological categories will never be specifiable in terms of necessary and sufficient conditions or shared underlying physical structures that produce clean boundaries. Biology blurs all lines and failure to embrace this unique feature has blocked attempts to reach consensus on the meaning of “life.” Thus, while the three classical accounts (thermodynamic, metabolic, and evolutionary) all fall short of offering a complete definition, their advocates fail to realize that they share the same view of life’s ultimate, functional hallmark: its uniquely rich adaptive capacity. I develop an account of life as adaptive capacity that sidesteps debates about the relative importance of specific mechanisms and the precise location of boundaries to bring the three classical accounts together under a shared conceptual framework.
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Notes
Of course, there are many who argue this is too simplistic.
There is a vast literature on natural kinds in general as well as their application in biology. For a very succinct account of the different ways to interpret natural kinds see Ereshefsky (2009).
Of course, the minute we give that example, modern readers will become uncomfortable and ask questions like, “Is reason really such an either-or proposition?” We know now that it is not, but Aristotle thought about reason, and scientific categories in general, as cleanly delineated. Certainly an essentialist intuition has, for millennia, guided attempts to explain how humans fit into the overall system of nature by emphasizing one or another supposedly unique attribute that sets us apart: reason, tool use, waging war, culture, and so on. Of course, we now know the truth is that humans, like any product of evolution, can’t be differentiated from other animals quite so clearly.
To be sure, this idea was a powerful one in many ways, but it caused problems as well. For example, the ancient Greeks’ thinking of planets as attached to perfect spheres, not because of any data about their circular motion, but because of a priori commitments to the perfect symmetry of the universe, set back astronomy many centuries.
Ironically, those who push clean boundaries are often explicitly anti-science. Creationists, for example, often insist the distinction between humans and nonhuman animals is a difference in kind rather than one of degree, despite all the evidence of a continuum.
The basic idea behind the biological species concept is that a species is defined by gene flow—who mates with whom. This works reasonably well for mammals like ourselves that engage in “ordinary sex,” but it just doesn’t apply to most microbes and many plants, which together constitute the vast bulk of living things on Earth. But humans like clean categories and we like simple mechanisms and we like critters that look like us, so many just ignore this basic fact.
Indeed, alien life may achieve much the same result with even more radical (from our point of view) structures.
Later we will formalize this idea in terms of screening-off relations.
This is, of course, a general expectation with a great many caveats.
Beatty’s (1995) Evolutionary Contingency Thesis is often cited to argue that we can not and should not predict how evolution will occur in different circumstances. Gould famously observes (1989) that evolution is like a videotape that, if it were to be replayed, would have a different ending each time. This is true as far as specific structures go, but we can expect convergence of function (all else being equal) when selection pressures are very similar.
I apologize for the extremely nontechnical explanation here, but I believe this accurately captures the essence of the concept. For a more precise account, see Prigogine (1973), Schneider and Sagan (2005), and Walker and Davies (2012). Chaisson (2002) talk about the use of free energy by nonequilibrium systems to increase complexity, though explicitly not in the context of defining life.
Indeed, there is an inherent tradeoff between generality and specificity such that no perfectly general account will be useful to someone involved in a more specific discipline (Smith 2016).
Walker and Davies (2012) suggest, for example, that the characteristics of certain physical systems set the stage for evolution by natural selection.
Of course, it is not at all clear exactly what “chemistry” means in this context—is a solid-state system with digital information “chemical” for example? (see Olson 1997).
Here again, people tend to assume that there must be a structural component (membrane) for this process, partly because such are easier to identify (and create). To some extent, this is another manifestation of the intuitive desire for clean boundaries, since it’s possible in principle to get the right kind of isolation without membranes at all, just as it’s possible to get speciation without reproductive isolation—it’s just much harder.
To put it another way, chemical properties are the genotype to evolutionary properties’ phenotype, in which case we only have life when both emerge together. This is similar to the debate about whether a trait is genetic or epigenetic—assuming there is a clean dichotomy is deeply confused (see Smith 1993).
It may even be that the transition between cheap homeostasis and true homeostasis is much harder to realize than metabolic advocates think—which could be part of the reason we have failed to realize such systems in the laboratory.
It is often referred to as “the NASA definition” even though NASA has never explicitly endorsed it.
Of course, it is possible to salvage the intuition that such individuals are alive, at least in some sense, by saying that they participate indirectly in evolution—populations are made up of individuals, after all, and without individual variation there would be no variation at the level of populations. At the very least, however, arguing that individuals are alive only indirectly does great violence to our intuitions. Indeed, a good case can be made (and is, by Aristotle) that individuals are in fact the paradigm cases for life, with other processes like populations being parasitic on them.
Consider, for example, that there is still debate about how to interpret the tests for life conducted on Mars by the Viking lander, which took certain types of soil chemistry as indicative of life. Everyone agrees that we did indeed encounter interesting soil chemistry, but now there is debate about whether or not this is a good indication of life (see Zyga 2016 for a recent overview of this debate).
This problem applies to evolutionary studies on Earth as well, providing ammunition to the creationists who demand visual proof of evolution in action.
To be fair, in principle there could be a proscriptive component to such analyses. It might be, for example, that what is being pursued is a “cluster definition,” if there truly is no set of necessary and sufficient conditions for life in general, but there is a set of insufficient but necessary components of an unnecessary but insufficient collection of conditions for life (see Mackie 1965). In that case, there would be an essence of life in a sense, it would just be that the essence is complex and not as easily captured as we would like. Most biologists do not discuss this kind of complexity, however, and the philosophers who pursue life relativism (Persson 2013) too quickly embrace despair about the possibility of a shared concept such as that developed in this article.
Not a natural science, anyway, though it could still count as a social science.
It has often been remarked that physical scientists have a much easier job than biologists (not to mention sociologists), because they have the luxury of dealing with a relatively small set of actors each having relatively constrained degrees of freedom. A star is certainly complex in one sense, but it’s much easier to explain its characteristics via a small set of critical principles, which is doubtless part of the reason physical science tends to produce categories with clean edges.
As someone who has spent considerable time in the trenches fighting creationists, I do not want to be misunderstood on this point.
Bedau (1996) comes closest to this idea when he argues that life is a system capable of “supple adaptation.” But he also endorses the idea that life is a property of populations rather than individuals, since only they participate in evolutionary processes.
It remains an open question how general this capacity for individuals to directly contribute to the creation of adaptive capacity is. For example, one might argue that a microbe capable of lateral gene transfer is performing a similar action to an intelligent human passing on an idea. Here, a genetic feature that arises within an individual is passed on directly to conspecifics (and sometimes even different species) without the mediation of population level selection.
Indeed, though it stretches our intuitions to the limit, life may not even be constrained to material systems: an evolving culture (however composed) could perhaps be considered alive in its own right.
Though, when it comes to species, even monophyly may not be necessary (Rieppel 2009).
A great deal of confusion results when people conflate the essence of a theoretical claim with its details. For example, all modern biologists consider evolution necessary to explain life, but they disagree on the relative importance (even sometimes the legitimacy) of specific evolutionary mechanisms. Thus creationists often (wrongly) claim that disagreement between advocates of gradualism and advocates of punctuated equilibria constitutes disagreement over the legitimacy of evolution in general. Even biologists and philosophers can fall prey to this, as when they associate “neo-Darwinian” evolution with specific mechanisms often associated with it (see my response to Cleland).
References
Adami C (2002) What is complexity? BioEssays 24:1085–1094
Baross J (2007) The limits of organic life in planetary systems. National Academies Press, Washington DC
Beatty J (1995) The evolutionary contingency thesis. In: Wolters G, Lennox J (eds) Concepts, theories and rationality in the biological sciences. University of Pittsburgh Press, Pittsburgh, pp 45–81
Bedau M (1996) The nature of life. In: Boden M (ed) The philosophy of artificial life. Oxford University Press, New York, pp 332–357
Boyd R (1991) Realism, anti-foundationalism and the enthusiasm for natural kinds. Philos Stud 61(1/2):127–148
Brandon R (1990) Adaptation and environment. Princeton University Press, Princeton
Brandon R, Antonovics J, Burian R et al (1994) Sober on Brandon on screening-off and the levels of selection. Philos Sci 61(3):475–486
Cavazzoni C, Chiarotti GL, Scandolo S et al (1999) Superionic and metallic states of water and ammonia at giant planet conditions. Science 283(5398):44–46
Chaisson E (2002) Cosmic evolution: the rise of complexity in nature. Harvard University Press, Cambridge
Chyba C, Mcdonald GD (1995) Annu Rev Earth Planet Sci 23:15–249
Ćirković M, Bradbury R (2006) Galactic gradients, postbiological evolution and the apparent failure of SETI. New Astron 11(8):628–639
Cleland C (2012) Life without definitions. Synthese 185:125–144
Cleland C, Chyba C (2002) Defining "life." Orig Life Evol Bios 32:387–393
Cleland C, Chyba C (2007) Does "life" have a definition? In: Sullivan W, Baross JA (eds) Planets and life: the emerging science of astrobiology. Cambridge University Press, Cambridge, pp 119–131
Cleland C, Copley S (2005) The possibility of alternative microbial life on earth. Int J Astrobiol 4:165–173
Cleland C, Zerella M (2013) What is life? In: Kampourakis K (ed) The philosophy of biology. Springer, New York, pp 31–48
Davies P (2011) The Eerie silence. Mariner Books, New York
Donoghue M (1985) A critique of the biological species concept and recommendations for a phylogenetic alternative. Bryologist 88(3):172–181
Ereshefsky M (2009) Natural kinds in biology. Routledge Encyclopedia of Philosophy, Taylor and Francis, https://www.rep.routledge.com/articles/thematic/natural-kinds-in-biology/v-1. Accessed 1 Sept 17
Fleischaker G (1990) Origins of life: an operational definition. Org Life Evol Biosph 20(2):127–137
Gould S (1989) Wonderful life. New York, Norton
Joyce G, Deamer D, Fleischaker G (1994) Origins of life: the central concepts. Jones and Bartlett, Boston
Korzeniewski B (2005) Confrontation of the cybernetic definition of living individual with the real world. Acta Biotheor 53:1–28
Koshland D (2002) The seven pillars of life. Science 295:2215–2216
Lauder G, Tytell E (2006) Hydrodynamics of undulatory propulsion. In: Shadwick R, Lauder G (eds) Fish biomechanics. Fish Physiology, vol 23. Academic Press, San Diego, pp 425–468.
Machery E (2011) Why I stopped worrying about the definition of life…and why you should as well. Synthese 185(1):145–164
Mackie J (1965) Causes and conditions. Am Philos Q 12:245–265
Margulis L, Sagan D (1995) What is life? Simon and Schuster, New York
Mariscal C (2016) Universal biology: assessing universality from a single example. In: Dick S (ed) The impact of discovering life beyond earth. Cambridge University Press, Cambridge, pp 113–126
Mayr E (2000) The biological species concept. In: Wheeler Q, Meier R (eds) Species concepts and phylogenetic history. Columbia University Press, New York, pp 17–29
McKay C (1998) Life in the planetary context. In: Woodward C, Shull M, Thronson H (eds) Origins, ASP conference series, vol 148, pp 449–456
Milligan T (2015) Nobody owns the moon. McFarland Publishing, Jefferson
Mishler B, Brandon R (1987) Individuality, pluralism, and the phylogenetic species concept. Biol Philos 2(4):397–414
Nixon K, Wheeler Q (1990) An amplification of the phylogenetic species concept. Cladistics 6(3):211–223
Oliver J, Perry R (2006) Definitely life but not definitively. Orig Life Evol Bios 36:515–521
Olson E (1997) The ontological basis of strong artificial life. Artif Life 3(1):29–39
Persson E (2013) Philosophical aspects of astrobiology. In: Dunér D, Pathermore J, Persson E, Holmberg G (eds) The history and philosophy of astrobiology. Cambridge Scholars Press, Newcastle upon Tyne, pp 29–48
Popa R (2004) Between necessity and probability. Springer, New York
Prigogine I (1973) Can thermodynamics explain biological order. Impact Sci Soc 23(3):159–169
Putnam H (1973) Meaning and reference. J Philos 70:699–711
Reichenbach H (1956) The direction of time. University of Los Angeles Press, Berkeley
Rieppel O (2009) Species monophyly. J Zoolog Syst Evolut Res 48(1):1–8
Schneider E, Sagan D (2005) Into the cool—energy flow, thermodynamics, and life. University of Chicago Press, Chicago
Schrödinger E (1944) What is life—the physical aspect of the living cell. Cambridge University Press, New York
Smith KC (1993) Marketing structuralism: reflections on the process structuralist critique of neo-darwinism. Rivista di Biologia/Biol Forum 86(3–4):578–579
Smith KC (2014) Manifest complexity: a foundational ethics for astrobiology? Space Policy 30(4):209–214
Smith KC (2016) Life is hard: countering definitional pessimism concerning the definition of life. Int J Astrobiol 15(4):277–289
Szostak J (2012) Attempts to define life do not help to understand the origin of life. J Biomol Struc Dynam 29(4):599–600
Toepfer G (2012) Teleology and its constitutive role for biology as the science of organized systems in nature. Stud Hist Philos Sci 43(1):113–119
Trifonov E (2012) Definition of life: navigation through uncertainties. J Biomol Struc Dynam 29(4):647–650
Wagner A (2013) Robustness and evolvability in living systems. Princeton University Press, Princeton
Walker S, Davies P (2013) The algorithmic origins of life. J R Soc Interface. https://doi.org/10.1098/rsif.2012.0869
Weaver W (1948) Science and complexity. Am Sci 36:536–544
Zyga L (2016) Did the 40-year-old Viking experiment discover life on Mars? Phys.org, https://phys.org/news/2016-10-year-old-viking-life-mars.html. Accessed 30 Aug 2017
Acknowledgements
The ideas in this article were inspired by numerous conversations with too many colleagues to mention. However, extensive comments provided on an earlier draft by Carlos Mariscal and Erik Persson were especially helpful. Any deficiencies that remain are, of course, entirely the fault of the author.
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Smith, K.C. Life as Adaptive Capacity: Bringing New Life to an Old Debate. Biol Theory 13, 76–92 (2018). https://doi.org/10.1007/s13752-017-0292-4
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DOI: https://doi.org/10.1007/s13752-017-0292-4