As experience clusters certain ideas into sets, so does the mind too, by its occult nature, cluster certain ideas into sets. These sets have various forms of connection. The simplest are sets of things all on one footing and agreeing in each belonging to the set. Such a set is a class. The clustering of ideas into classes is the simplest form which the association of ideas by the occult nature of ideas, or of the mind, can take.
…a class is a set of objects comprising all that stand to one another in a special relation of similarity.
Every class is constituted and held together by a concept or idea expressed in its definition.
A class […] is the total of whatever objects there may be in the universe which are of a certain description.
…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.