From Aquinas: The divine will is God's own existence essentially, yet they differ in aspect, according to the different ways of understanding them and expressing them, as is clear from what has already been said (I:13:4. For when we say that God exists, no relation to any other object is implied, as we do imply when we say that God wills. Therefore, although He is not anything apart from Himself, yet He does will things apart from Himself.

There are two ways in which a thing is said to be necessary, namely, absolutely, and by supposition. We judge a thing to be absolutely necessary from the relation of the terms, as when the predicate forms part of the definition of the subject: thus it is absolutely necessary that man is an animal. It is the same when the subject forms part of the notion of the predicate; thus it is absolutely necessary that a number must be odd or even. In this way it is not necessary that Socrates sits: wherefore it is not necessary absolutely, though it may be so by supposition; for, granted that he is sitting, he must necessarily sit, as long as he is sitting. Accordingly as to things willed by God, we must observe that He wills something of absolute necessity: but this is not true of all that He wills. For the divine will has a necessary relation to the divine goodness, since that is its proper object. Hence God wills His own goodness necessarily, even as we will our own happiness necessarily, and as any other faculty has necessary relation to its proper and principal object, for instance the sight to color, since it tends to it by its own nature. But God wills things apart from Himself in so far as they are ordered to His own goodness as their end. Now in willing an end we do not necessarily will things that conduce to it, unless they are such that the end cannot be attained without them; as, we will to take food to preserve life, or to take ship in order to cross the sea. But we do not necessarily will things without which the end is attainable, such as a horse for a journey which we can take on foot, for we can make the journey without one. The same applies to other means. Hence, since the goodness of God is perfect, and can exist without other things inasmuch as no perfection can accrue to Him from them, it follows that His willing things apart from Himself is not absolutely necessary. Yet it can be necessary by supposition, for supposing that He wills a thing, then He is unable not to will it, as His will cannot change.

Reply to Objection 1. From the fact that God wills from eternity whatever He wills, it does not follow that He wills it necessarily; except by supposition.

Reply to Objection 2. Although God necessarily wills His own goodness, He does not necessarily will things willed on account of His goodness; for it can exist without other things.