Natural Kind

If two kinds, A and B, are so related that of whatever singulars A could possibly be true, B would necessarily also be true, then A is said to involve B.

This necessity may be of any of the modes of necessity. In particular, if A involves B because of the definitions, or very ideas, of the two kinds, A is said essentially to involve, or, in other words, to imply B. A kind all whose singulars seem, according to experience, normally to belong to other kinds not implied in the former kind, is called (especially if the other kinds are numerous) a natural kind.