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