that no list of purely physical characteristics can settle whether a given entity is alive. The usual candidates – growth, reproduction, homeostasis, orderliness, self-motion, energy exchange, responsiveness to stimuli, etc., etc., – always turn out, as Thompson shows in detail, to be relevant only when they are the right sorts of growth, reproduction, homeostasis, and so on; and there seems to be no noncircular way of specifying the right sort, i.e., no way that doesn’t appeal to the very biological categories one was trying to define. Just as praxeological concepts are definable only in terms of other praxeological concepts, so biological concepts are definable only in terms of other biological concepts.
What is the difference between living and non-living things, then? Thompson suggests that it consists in the applicability or otherwise of a certain form of description he calls “natural-historical judgments” – namely, statements of the following familiar sort:
“The S is (or has, or does) F” – “The domestic cat has four legs, two eyes, two ears, and guts in its belly”; “The Texas bluebonnet harbors nitrogen-fixing microbes in certain nodes on its roots”; “The tallow finch breeds in spring, attracting its mate with such and such song” ....31
Such descriptions are more slippery than they might seem. They cannot mean that, say, all domestic cats have four legs (some will suffer accidents or birth defects) or that all tallow finches breed in spring (many will meet their deaths in youthful virginity). One might suppose that natural-historical judgments are yet another instance of logical preponderance, and really mean only that most S’s are or have or do F; but as Thompson points out, that won’t work either. If some worldwide calamity deprived the majority of cats of their legs, we would still say that the domestic cat has four legs. And there are plenty of species (particularly among fish and insects) in which the majority of offspring do not survive to breeding age; yet we still say of them such things as the such-and-such breeds in spring, meaning that it does so in the normal case – in some non-statistical
31 Michael Thompson, Life and Action: Elementary Structures of Practice and Practical Thought (Harvard University Press, 2008), pp. 64-65.
R. T. Long – Wittgenstein, Praxeology, and Frege’s Three Realms – p. 21