"Life, like science and art, is a theory about the world: a theory that in our case takes bodily form. By a succession of adaptations, most of which are favourable and none of which are lethal, living things have invested in particular expectations about the future course of their environments. If those theories are good enough, then life will prosper and multiply; but if they are outmoded by changing conditions, their embodiments will dwindle and perish."