A character which is not sometimes true and sometimes false of the same singular is a kind. A kind may not exist at all; or it may exist in but one sole singular, which the old logics used to say was the case with the kind called sun. [—]

I consider a kind to be an ens rationis, although that may be open to dispute, at least as regards some kinds; but there can, I think, be no doubt that a class is an ens rationis. For a class, unlike a kind, is not a character, but is the totality of all those singulars that possess a definite existent character, which is the essential character of the class.