Summa contra Gentiles1.84 Chapter LXXXIV. That God's will is not of things impossible in themselves

Hence it is clear that God's will cannot be of things that are impossible in themselves.

For the like are those which imply a contradiction in themselves: for instance that a man be an ass, which implies that rational is irrational. Now that which is incompatible with a thing excludes some of those things which are required for that thing: for instance, to be an ass excludes man's reason. If, then, He wills necessarily the things that are required for those He is supposed to will, it is impossible that He will those that are incompatible with them. Hence it is impossible for Him to will things that are simply impossible.

Again. As was proved above, God, by willing His own being, which is His own goodness, wills all things as bearing a likeness to Him. Now in so far as a thing is incompatible with the notion of being as such, it cannot retain a likeness to the first, that is, the divine being, which is the source of being. Wherefore God cannot will that which is incompatible with the notion of being as such. Now just as irrationality is incompatible with the notion of man as such, so is it incompatible with the notion of being as such, that anything be at the same time a being and a non-being. Hence God cannot will affirmation and negation to be true at the same time. Yet this is implied in everything which is in itself impossible, that it is incompatible with itself, in as much as it implies a contradiction. Therefore God's will cannot be of things impossible in themselves.

Moreover. The will is only of some understood good. Wherefore that which is not an object of the intellect, cannot be an object of the will. Now things in themselves impossible are not an object of understanding, since they imply a contradiction, except perhaps through an error of one who understands not the property of things: and this cannot be said of God. Therefore things in themselves impossible cannot be an object of God's will.

Further. According as a thing is related to being, so is it related to goodness. But impossibles are things that cannot be. Therefore they cannot be good. Neither therefore can they be willed by God, Who wills only the things that are or can be good.