Life is hard: countering definitional pessimism concerning the definition of life

Abstract

AbstractCleland and Chyba published a classic piece in 2002 that began a movement I call definitional pessimism, where it is argued that there is no point in attempting anything like a general definition of life. This paper offers a critical response to the pessimist position in general and the influential arguments offered by Cleland and her collaborators in particular. One such argument is that all definitions of life fall short of an ideal in which necessary and sufficient conditions produce unambiguous categorizations that dispose of all counterexamples. But this concept of definition is controversial within philosophy; a fact that greatly diminishes the force of the admonition that biologists should conform to such an ideal. Moreover, biology may well be fundamentally different from logic and the physical sciences from which this ideal is drawn, to the point where definitional conformity misrepresents biological reality. Another idea often pushed is that the prospects for definitional success concerning life are on a par with medieval alchemy's attempts to define matter – that is, doomed to fail for lack of a unifying scientific theory. But this comparison to alchemy is both historically inaccurate and unfair. Planetary science before the discovery of the first exoplanets offers a much better analogy, with much more optimistic conclusions. The pessimists also make much of the desirability of using microbes as models for any universal concept of life, from which they conclude that certain types of ‘Darwinian’ evolutionary definitions are inadequate. But this argument posits an unrealistic ideal, as no account of life can both be universal and do justice to the sorts of precise causal mechanisms microbes exemplify. The character of biology and the demand for universality in definitions of life thus probably accords better with functional rather than structural categories. The bottom line is that there is simply no viable alternative, either pragmatically or theoretically, to the pursuit of definitions. If nothing else, the empirical data the pessimists demand will be a very long time coming and scientists will of necessity continue to employ definitions of life in the interim. Chastising them for this will only drive their ideas underground where they can escape critical analysis, making the problems caused by problematic conceptions of life worse.