 It seems that those who are committed to there being something which accounts for the numerical distinctness of individuals must say that there is nothing that accounts for the numerical distinctness of the distinctness-makers. The only alternative would be to introduce some further thing to account for their distinctness, and so on; but this results in an infinite regress, which, as well as being ontologically bloated, appears to be vicious, since we can never grasp the full account of what makes Socrates and Callias distinct. Both sides agree that explanation must stop somewhere, but they differ over where it is appropriate to stop: is it a basic, inexplicable fact that Socrates is numerically distinct from Callias, or that their matter is distinct? (See Markosian 2008, §8, for a contemporary discussion of this question.) At any rate, even if it is difficult to prove that there is an important metaphysical question here, the traditional interpretation of Aristotle is that he thinks there is.