columns of words; the middlemost (in boldface) contains the series of genera and species, and we can take it as analogous to the trunk of a tree. The extremes (the terms that jut out to the left and right), containing the differentiae, we can take as analogous to the branches of a tree: The diagram shows the highest genus to be substance. (Whether substance is a highest genus, really, is not in question here: right now we are only going to discuss what the diagram shows, not whether what it shows is true or false.) The technical term for a highest substance is summum genus. So, substance is the summum genus as far as this diagram goes. The diagram shows that the genus substance has two differentia, namely, "thinking" and "extended". This indicates that there are two species of the genus substance, thinking substance and extended substance. The diagram does not give a term for the species of thinking substance (this would be "mind"), but it does give the term for the species of extended substance, namely, body. That is, body is a species of the genus substance; body is that species of the genus substance that is extended. Now that we have seen body as a species of substance, we treat body as a genus itself. As a genus, it has two differentia of its own, inanimate and animate. So, there are two species of body, inanimate body and animate body. The diagram does not tell us what the term for inanimate body is, but it indicates a term for animate body, namely, animal. Animal is an animate species of the genus body. And, again, now that we have looked at animal as a species of the genus body, we look at animal now as a genus and consider its differentia, which are shown on the diagram to be irrational and rational. Thus, according to the diagram there are two species of the genus animal, irrational animal and rational animal. We are not told by the diagram what a term for irrational animal is, but the diagram indicates that a rational animal is a human. Thus, human is a rational species of the genus animal. Beneath human, however, there are no further species. "This" and "that" if they are considered differentiae, are of a special kind that map the species human not onto a new species but onto particular