 It might seem that the answer to this question is obvious: of course there is a highest kind — being. After all, someone might argue, everything exists. So the class that contains all and only beings must be the class with the greatest possible extension. In the Metaphysics, however, Aristotle argues that being is not a genus (998b23, 1059b31). According to Aristotle, every genus must be differentiated by some differentia that falls outside that genus. Hence, if being were a genus, it would have to be differentiated by a differentia that fell outside of it. In other words, being would have to be differentiated by some non-being, which, according to Aristotle, is a metaphysical absurdity. Although he does not explicitly make this claim, Aristotle’s argument, if cogent, would generalize to any proposal for a single highest kind. Hence, he does not think that there is one single highest kind. Instead, he thinks that there are ten: (1) substance; (2) quantity; (3) quality; (4) relatives; (5) somewhere; (6) sometime; (7) being in a position; (8) having; (9) acting; and (10) being acted upon (1b25–2a4). I shall discuss the first four of these kinds in detail in a moment. But doing so will take us into matters that, while interesting, nonetheless distract from the general nature of the scheme. So I will first discuss some of the general structures inherent in Aristotle’s second system of classification, and then proceed to a more detailed discussion.