As there is Intellect in God, it follows that there is also Will in Him. For as natural things have their being through form, to which they are so related that if they have it not they tend towards it, and if they have it they rest in it, so the intellectual nature either tends to or rests in the good it apprehends. This belongs to the will; where, therefore, we find intellect we find also will, as animal strength exists where there is sensation. Since, therefore, there is Intellect in God, and His Intellect is His own very Existence, so also is His Will.
And God wills others besides Himself. For we find in every perfect agent, not only an inclination towards its own good, either to gain it or rest in it, but an inclination to diffuse that good as much as possible to others; and from this action of the perfect agent producing an effect similar to itself, we learn what best befits the Divine Will. Thus God wills Himself as the End, and others to the end; it is, therefore, fitting that these should participate in the Divine Goodness, for although God's Goodness suffices to Himself, He nevertheless wills others by reason of His Goodness.
God wills something of absolute necessity, for He has a necessary inclination towards His own Goodness as His proper object; in this way we also will our own beatitude. Other things, indeed, God wills that they may be ordered to His Goodness as their end; but what is willed for an end is not willed of necessity, unless it be such that the end could not be had without it, as life without food. Since, therefore, the Goodness of God can exist without anything else, and is in itself perfect, it follows that He does not will other things for Himself by absolute necessity, but only hypothetically. By hypothesis, however, what God wills He cannot not will, for His Will cannot change.
That the Will of God is the cause of things may be proved in three ways: first, because, in respect of every natural agent, the end and the means are predetermined by some intelligence, and, therefore, the agent by intelligence is prior to the natural agent; and God is the First Agent. Secondly, every natural agent has a determinate being, and one and the same mode of action, unless prevented; and since the Divine Being is Infinite, and contains in Itself every perfection, God's natural action would be infinite in producing being, which is impossible; therefore He acts according to the determination of His Will. Thirdly, effects proceed from the acting cause according as they pre-exist in it, and since the Being of God is His Intelligence, such effects must proceed according to the mode of His Intelligence and Will.
Nor is there any cause for His Will, for He understands all by one act in His Essence, and by one act He wills all in His Goodness; therefore, as in God His understanding of the cause is not the cause of His understanding the effects, but the effects are understood by Him in the cause, so His willing the end is not the cause of His willing those things which are ordained to the end; nevertheless He wills that the things which are for the end should be ordered to it.
And the Will of God is always fulfilled. Because, as nothing can fall away from the universal form, yet may do so from some particular form, so with causes proceeding from agents; something may happen outside the order of a particular cause, but not outside that of the universal cause; since, therefore, the Will of God is the universal Cause of all things, it must necessarily be always fulfilled. Thus the sinner who departs from the Divine Will, as far as it is in his power to do so, by sinning, falls into the order of the Divine Will when by the justice of God he is punished.
Further, the Will of God is immutable. For change could not occur except by a presupposed mutation, either with regard to His thoughts or to the dispositions of the substance of His Will; but the Substance of God is equally immutable with His knowledge, therefore so also is His Will. Thus Scripture speaks according to our human ideas when it says: "I repent," etc., or when something is represented as being in the future according to particular causes which, according to the universal Cause, was nevertheless not in the future, as in Isaiah 38: "Take order with thy house," etc.
God does not, however, impose all that He wills by a necessary will, but by His efficacious Will things are brought about as He wills they should be, some of necessity and some contingently, that there may be order in everything for the carrying out of the whole. Certain effects are fitted, therefore, to necessary causes which cannot fail, and others to contingent causes which are detectible, out of which contingent effects proceed.
Neither does God in any way will the evil of sin, although He wills accidentally the evil of natural defect and the evil of punishment because of some good attached to it. Thus, willing justice, He wills punishment, and willing to preserve the order of nature He wills that certain natural things should decay. But evil, as such, cannot attract any one's desire, natural, animal or intellectual.
And God has free Will in those things which He does not will of necessity. For He wills other things besides Himself, and that freely; and this Will, if considered in itself, is called His Will of Complacency. Metaphorically, also, the manifestation of the will by signs is called will.
The signs, commonly received, are: prohibition, precept, counsel, operation and permission; for that is called a sign by which we are accustomed to express our will. Thus permission and operation have respect to "the present, prohibition and precept to the future, while counsel has reference to the superabundance of good.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni