Summa contra Gentiles1.17 Chapter XVII. That in God there is no matter

From this it follows that God is not matter.

For matter, such as it is, is in potentiality.

Again. Matter is not a principle of activity: wherefore, as the Philosopher puts it, efficient and material causes do not coincide. Now, as stated above, it belongs to God to be the first efficient cause of things. Therefore He is not matter.

Moreover. For those who referred all things to matter vas their first cause, it followed that natural things exist by chance: and against these it is argued in 2 Phys. Therefore if God, Who is the first cause, is the material cause of things, it follows that all things exist by chance. i Further. Matter does not become the cause of an actual thing, except by being altered and changed. Therefore if God is immovable, as proved above, He can nowise be a cause of things as their matter.

The Catholic faith professes this truth, asserting that God created all things not out of His substance, but out of nothing.

The ravings of David of Dinant are hereby confounded, who dared to assert that God is the same as primary matter, because if they were not the same, they would needs differ by certain differences, and thus they would not be simple: since in that which differs from another thing by a difference, the very difference argues composition. Now this proceeded from his ignorance of the distinction between difference and diversity. For as laid down in 10 Metaph. a thing is said to be different in relation to something, because whatever is different, differs by something, whereas things are said to be diverse absolutely from the fact that they are not the same thing. Accordingly we must seek for a difference in things which have something in common, for we have to point to something in them whereby they differ: thus two species have a common genus, wherefore they must needs be distinguished by differences. But in those things which have nothing in common, we have not to seek in what they differ, for they are diverse by themselves. For thus are opposite differences distinguished from one another, because they do not participate in a genus as a part of their essence: and consequently we must not ask in what they differ, for they are diversified by their very selves. Thus too, God and primary matter are distinguished, since, the one being pure act and the other pure potentiality, they have nothing in common.