Every class has its definition, which is an idea; but it is not every class where the existence, that is, the occurrence in the universe of its members is due to the active causality of the defining idea of the class. That circumstance makes the epithet natural particularly appropriate to the class. The word natura evidently must originally have meant birth; although even in the oldest Latin it very seldom bears that meaning.
A class […] is the total of whatever objects there may be in the universe which are of a certain description. What if we try taking the term “natural,” or “real, class” to mean a class of which all the members owe their existence as members of the class to a common final cause? This is somewhat vague; but it is better to allow a term like this to remain vague, until we see our way to rational precision.
…a natural class [is] a family whose members are the sole offspring and vehicles of one idea, from which they derive their peculiar faculty…