 According to the traditional interpretation, these lines are saying that Socrates and Callias are numerically distinct because of their matter, not their form, and on the face of it this is the clearest example of Aristotle affirming that matter is the principle of individuation. We can adopt an alternative reading, however, if we suppose that “different” means not numerically distinct, but qualitatively different. In that case, the passage could be making an epistemological claim about how we discern Socrates and Callias: suppose Callias is pale and Socrates dark; they are different, but not different in form; they differ because of their matter, since pallor and darkness primarily qualify their skin, i.e., part of their body.