Encyclopedia of Philosophy: Matter, Form, Substance, and Accident (Aquinas)

In metaphysics Aquinas was a faithful follower of Aristotle – though not a slavish one, as the example of the eternal universe illustrates. He accepted the analysis of material bodies in terms of matter and form, and the thesis that change is to be explained as the reception of different successive forms in the same matter. He also accepted the Aristotelian doctrine that matter is the principle of individuation: if two pebbles on the beach resemble each other in every possible way, they will not differ at all in form, but they will be two pebbles and not one pebble because they are two different chunks of matter.  These Aristotelian theses gave rise to problems in connection with the angels of which the Bible speaks and which Christian tradition had come to regard as non-corporeal. Aquinas regarded as implausible Bonaventure's suggestion that angels too contained matter, but matter that was spiritual. Instead, he regarded angels as being pure immaterial forms. But if matter is the principle of individuation, how can there be more than one immaterial angel? Aquinas' answer was that each angel was a form of a different kind, a species all to himself. So Michael and Gabriel would differ from each other not as Peter differs from Paul, but as a sheep differs from a cow.