A quote
Certain of the Stoics regard the primary genus as the “something.” I shall add the reasons they give for their belief; they say: “in the order of nature some things exist, and other things do not exist. And even the things that do not exist are really part of the order of nature. What these are will readily occur to the mind, for example centaurs, giants, and all other figments of unsound reasoning, which have begun to have a definite shape, although they have no bodily consistency.”
Seneca·Letter 58 — On Being (§15)·trans. Gummere