A physiologist... can certainly offer a definition of life; but this will only be an interim report on the progress of physiology to date. For him, as for the beginner, it is the nature of physiology that is relatively certain; it is the nature of life that is relatively vague.

The New Leviathan: Or Man, Society, Civilization and Barbarism, Part I, Chapter I. Aphorism I.47 (p. 3), At The Clarendon Press. 1942