              Further, in Metaphysics vii 17 Aristotle offers a suggestive argument to the effect that matter alone cannot be substance. Let the various bits of matter belonging to Socrates be labeled as a, b, c, …, n. Consistent with the non-existence of Socrates is the existence of a, b, c, …, n, since these elements exist when they are spread from here to Alpha Centauri, but if that happens, of course, Socrates no longer exists. Heading in the other direction, Socrates can exist without just these elements, since he may exist when some one of a, b, c, …, n is replaced or goes out of existence. So, in addition to his material elements, insists Aristotle, Socrates is also something else, something more (heteron ti; Met. 1041b19–20). This something more is form, which is ‘not an element…but a primary cause of a thing’s being what it is’ (Met.  1041b28–30). The cause of a thing’s being the actual thing it is, as we have seen, is form. Hence, concludes Aristotle, as the source of being and unity, form is substance. 