Summa contra Gentiles1.68 Chapter LXVIII. That God knows the movements of the will

In the next place we must show that God knows our mind's thoughts and our secret wills.

For everything, in whatever way it exists, is known by God, in as much as He knows His essence, as we have shown above. Now some things are in the soul, and some in things outside the soul. Wherefore God knows all these differences of things and whatever is contained under them. Now the things in the soul are those that are in our will or our thought. It remains, therefore, that God knows what we have in our thoughts and wills.

Moreover. God so knows other things in knowing His essence, as effects are known through their cause being known. Accordingly by knowing His essence God knows all the things to which His causality extends. Now this extends to the works of the intellect and will: for, since every thing acts by its form which gives the thing some kind of being, it follows that the highest source of all being, from which also every form is derived, must be the source of all operation; because the effects of second causes are to be referred in a still higher degree to first causes. Therefore God knows both the thoughts and the affections of the mind.

Again. Even as His being is first and consequently the cause of all being, so His act of intelligence is first, and consequently the cause of all intellectual operation. Wherefore just as God by knowing His being knows the being of everything, so by knowing His act of intelligence and will He knows every thought and will.

Further. God knows things not only as existing in themselves, but also as existing in their causes, as proved above: for He knows the relation between cause and effect. Now the products of art are in the craftsman through the intellect and will of the craftsman, even as natural things are in their causes through the powers of the causes: for, just as natural things liken their effects to themselves by their active powers, so the craftsman by his intellect gives his handiwork the form whereby it is likened to his art. It is the same with all things done of set purpose. Therefore God knows both our thoughts and our wills.

Again. Intelligible substances are no less known to God than sensible substances are known to Him or to us: since intelligible substances are more knowable, for as much as they are more actual. Now the informations and inclinations of sensible substances are known both to God and to us. Consequently, since the soul's thought results from its being informed, and since its affection is its inclination towards something—for even the inclination of a natural thing is called its natural appetite—it follows that God knows our secret thoughts and affections.

This is confirmed by the testimony of Holy Writ. For it is said in the psalm: The searcher of hearts and reins is God: and (Prov. xv. 11): Hell and destruction are before the Lord: how much more the hearts of the children of men; and (Jo. ii. 25): He knew what was in man.

The dominion which the will exercises over its own acts, 1 Ch. lxvi. 2 Ps. vii. 10.

and by which it is in its power to will and not to will, removes the determination of the power to one thing, and the violence of a cause acting from without: but it does not exclude the influence of a higher cause from which it has being and action. Thus causality remains in the first cause which is God, in respect of the movements of the will; so that God is able to know them by knowing Himself.