Objection 1. It would seem that the will is not moved by anything exterior. For the movement of the will is voluntary. But it is essential to the voluntary act that it be from an intrinsic principle, just as it is essential to the natural act. Therefore the movement of the will is not from anything exterior.
Objection 2. Further, the will cannot suffer violence, as was shown above (Question 6, Article 4). But the violent act is one "the principle of which is outside the agent" [Aristotle, Ethic. iii, 1. Therefore the will cannot be moved by anything exterior.
Objection 3. Further, that which is sufficiently moved by one mover, needs not to be moved by another. But the will moves itself sufficiently. Therefore it is not moved by anything exterior.
On the contrary, The will is moved by the object, as stated above (Article 1). But the object of the will can be something exterior, offered to the sense. Therefore the will can be moved by something exterior.
I answer that, As far as the will is moved by the object, it is evident that it can be moved by something exterior. But in so far as it is moved in the exercise of its act, we must again hold it to be moved by some exterior principle.
For everything that is at one time an agent actually, and at another time an agent in potentiality, needs to be moved by a mover. Now it is evident that the will begins to will something, whereas previously it did not will it. Therefore it must, of necessity, be moved by something to will it. And, indeed, it moves itself, as stated above (Article 3), in so far as through willing the end it reduces itself to the act of willing the means. Now it cannot do this without the aid of counsel: for when a man wills to be healed, he begins to reflect how this can be attained, and through this reflection he comes to the conclusion that he can be healed by a physician: and this he wills. But since he did not always actually will to have health, he must, of necessity, have begun, through something moving him, to will to be healed. And if the will moved itself to will this, it must, of necessity, have done this with the aid of counsel following some previous volition. But this process could not go on to infinity. Wherefore we must, of necessity, suppose that the will advanced to its first movement in virtue of the instigation of some exterior mover, as Aristotle concludes in a chapter of the Eudemian Ethics (vii, 14).
Reply to Objection 1. It is essential to the voluntary act that its principle be within the agent: but it is not necessary that this inward principle be the first principle unmoved by another. Wherefore though the voluntary act has an inward proximate principle, nevertheless its first principle is from without. Thus, too, the first principle of the natural movement is from without, that, to wit, which moves nature.
Reply to Objection 2. For an act to be violent it is not enough that its principle be extrinsic, but we must add "without the concurrence of him that suffers violence." This does not happen when the will is moved by an exterior principle: for it is the will that wills, though moved by another. But this movement would be violent, if it were counter to the movement of the will: which in the present case is impossible; since then the will would will and not will the same thing.
Reply to Objection 3. The will moves itself sufficiently in one respect, and in its own order, that is to say as proximate agent; but it cannot move itself in every respect, as we have shown. Wherefore it needs to be moved by another as first mover.
Source: Summa Theologica (New Advent)