 There is a difficulty for the idea that matter can act as the principle of individuation, which arises out of the following problem that can be raised for Aristotle’s hylomorphism (see Fine 1994). It seems that two substances, e.g., Socrates and Callias, may have numerically the same matter at different times; that it is possible (however unlikely) for all and only the particular elements that now compose Socrates to end up composing Callias at some later date. In such a case, Socrates and Callias would have the same matter, albeit at different times. Moreover, both being human beings, they would have the same form. But they themselves are compounds of matter and form, so if their matter and form are numerically the same, they must themselves be numerically the same.