The Principles of Mathematics (1903)

§ 50

Two concepts have, in addition to the numerical diversity which belongs to them as terms, another special kind of dviersity which may be called conceptual. This may be characterized by the fact that two propositions in which the concepts occur otherwise than as terms, even if, in all other respects, the two propositions are identical, yet differ in virtue of the fact that the concepts which occur in them are conceptually diverse. Conceptual diversity implies numerical diversity, but the converse implication does not hold, since not all terms are concepts. Numerical diversity, as the name implies, is the source of plurality, and conceptual diversity is less important in mathematics. But the whole possibility of making different assertions about a given term or set of terms depends upon conceptual diversity, which is therefore fundamental in general logic.(§ 50 ¶ 1)