Letter 58 — On Being (§8)

“But if I must do so, I shall render it by quod est. There are six ways in which Plato expresses this idea, according to a friend of ours, a man of great learning, who mentioned the fact to-day. And I shall explain all of them to you, if I may first point out that there is something called genus and something called species. For the present, however, we are seeking the primary idea of genus, on which the others, the different species, depend, which is the source of all classification, the term under which universal ideas are embraced. And the idea of genus will be reached if we begin to reckon back from particulars; for in this way we shall be conducted back to the primary notion.”