The Commens Dictionary
Quote from ‘The Fourth Curiosity’
…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. Should observation show that two classes having different essential characters embraced the very same singulars, then since it is the singulars, and not the kinds, that constitute the existence of the class, we should say that the two classes, though entitatively, that is, in their possibilities, they were diverse, were yet existentially one. Such, I think, is the modern notion of a class, though I must confess that it appears to me to be rather hazy. The characters which go to define a class are not necessarily permanent characters of the singulars, as a kind is. On the contrary we speak with perfect propriety of the class of human males between the ages of fourteen and twenty-one, though there is evidently no such kind.