 One might think that one could respond to this argument by insisting that there is an answer to the question what makes Socrates’ matter numerically distinct from Callias’ matter: it is the matter itself. If matter can explain the distinctness of individual substances, why should it not also explain its own distinctness from other matter? Whether or not this move is legitimate will depend on which facts are and which facts are not in need of explanation but may correctly be assumed to be primitive. The problem is that “this matter is distinct from that matter because it is this matter” seems to be a very similar sort of explanation to “Socrates is distinct from Callias because he is Socrates”—both are cases of x explaining its own distinctness from y. Either both should count as adequate explanations or neither should. But the advocate of matter as principle of individuation adopted this view precisely because she found this sort of explanation unsatisfactory, or not an explanation at all. Therefore this response does not seem to be open to her.