The purpose of this paper is to provide a methodological rather than, strictly speaking, a philosophical discussion of its subject, the logic of concept formation in psychology. But even a treatment of this kind cannot entirely avoid matters of a more general nature, some of them logical, some epistemological. By insisting on the limitations of this essay I merely wish to caution the reader in three respects. First, those more general matters, logical and epistemological, will be kept at a minimum. Second, no attempt will be made to state them with the degree of precision and all the qualifications which are in order in a paper that addresses itself exclusively to logical analysts. Third, I shall for the most part content myself with stating them, without defending them in the way and in the sense in which a technical philosopher who speaks to his colleagues must defend what he asserts.