 This genus/species hierarchy is far from complete — Aristotle’s biological treatises contain a remarkably rich taxonomy of animals that is neither captured nor indeed obviously commensurate with the division into irrational and rational animals— but it does nicely illustrate the general structure of Aristotle’s categories. The lowest species in this taxonomy give way to kinds of increasing generality until the highest kind, substance is reached. Moreover, there is something rather intuitive about the idea that members of natural kinds are a fundamental type of entity in the world and hence that there is a system of kinds of increasing generality to which each such entity belongs. Of course, someone might think that some kind stands above substance. But it is not clear what such a kind would be except being, or perhaps the even more general kind thing; and as I have already said, not only does Aristotle reject the idea that being is a genus, but it is difficult to see what the relevant sense of thing is, if this is not simply another word for substance.