(CG 1/036). The Propositions which our Understanding forms of God are not vo... (Book 1: Of God As He Is In Himself) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

EVERY good thing, that is not its own goodness, is called good by participation. But what is called good by participation presupposes something else before itself, whence it has received the character of goodness. This process cannot go to infinity, as there is no processus in infinitum in a series of final causes: for the infinite is inconsistent with any end, while good bears the character of an end. We must therefore arrive at some first good thing, which is not good by participation in reference to anything else, but is good by its own essence; and that is God.

4. What is, may partake of something; but sheer being can partake of nothing. For that which partakes, is potentiality: but being is actuality. But God is sheer being, as has been proved (Chap. XXII) : He is not then good by participation, but essentially so.

5. Every simple being has its existence and what it is, in one: if the two were different, simplicity would be gone. But God is absolute simplicity, as has been shown (Chap. XVIII): therefore the very goodness that is in Him is no other than His own very self.

The same reasoning shows that no other good thing is its own goodness: wherefore it is said: None is good but God alone (Mark x, 18; Luke xviii, 19).