 In addition to positing ten highest kinds, Aristotle also has views about the structure of such kinds. Each kind is differentiated into species by some set of differentiae. In fact, the essence of any species, according to Aristotle, consists in its genus and the differentia that together with that genus defines the species. (It is for this reason that the highest kinds are, strictly speaking, indefinable — because there is no genus above a highest kind, one cannot define it in terms of its genus and a differentia.) Some of the species in various categories are also genera — they are, in other words, differentiated into further species. But at some point, there is a lowest species that is not further differentiated. Under these species, we can suppose, fall the particulars that belong to that species.