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In this article, St. Thomas limits himself to a demonstration of the revealed truth. He argues from three previously demonstrated truths.
First, we may know that God is immutable from the fact that He is actus purus. For, that which is mutable is in potentiality (for, motion is nothing other than a change from potency to act).
Second, we may know that God is immutable from the fact that He is simple. For, that which is mutable is changed in one part and remains in another (else, there would not be a single subject that is changed).