Objection 1. It would seem that the human will is moved by a heavenly body. For all various and multiform movements are reduced, as to their cause, to a uniform movement which is that of the heavens, as is proved in Phys. viii, 9. But human movements are various and multiform, since they begin to be, whereas previously they were not. Therefore they are reduced, as to their cause, to the movement of the heavens, which is uniform according to its nature.
Objection 2. Further, according to Augustine (De Trin. iii, 4) "the lower bodies are moved by the higher." But the movements of the human body, which are caused by the will, could not be reduced to the movement of the heavens, as to their cause, unless the will too were moved by the heavens. Therefore the heavens move the human will.
Objection 3. Further, by observing the heavenly bodies astrologers foretell the truth about future human acts, which are caused by the will. But this would not be so, if the heavenly bodies could not move man's will. Therefore the human will is moved by a heavenly body.
On the contrary, Damascene says (De Fide Orth. ii, 7) that "the heavenly bodies are not the causes of our acts." But they would be, if the will, which is the principle of human acts, were moved by the heavenly bodies. Therefore the will is not moved by the heavenly bodies.
I answer that, It is evident that the will can be moved by the heavenly bodies in the same way as it is moved by its object; that is to say, in so far as exterior bodies, which move the will, through being offered to the senses, and also the organs themselves of the sensitive powers, are subject to the movements of the heavenly bodies.
But some have maintained that heavenly bodies have an influence on the human will, in the same way as some exterior agent moves the will, as to the exercise of its act. But this is impossible. For the "will," as stated in De Anima iii, 9, "is in the reason." Now the reason is a power of the soul, not bound to a bodily organ: wherefore it follows that the will is a power absolutely incorporeal and immaterial. But it is evident that no body can act on what is incorporeal, but rather the reverse: because things incorporeal and immaterial have a power more formal and more universal than any corporeal things whatever. Therefore it is impossible for a heavenly body to act directly on the intellect or will. For this reason Aristotle (De Anima iii, 3) ascribed to those who held that intellect differs not from sense, the theory that "such is the will of men, as is the day which the father of men and of gods bring on" [Odyssey xviii. 135 (referring to Jupiter, by whom they understand the entire heavens). For all the sensitive powers, since they are acts of bodily organs, can be moved accidentally, by the heavenly bodies, i.e. through those bodies being moved, whose acts they are.
But since it has been stated (2) that the intellectual appetite is moved, in a fashion, by the sensitive appetite, the movements of the heavenly bodies have an indirect bearing on the will; in so far as the will happens to be moved by the passions of the sensitive appetite.
Reply to Objection 1. The multiform movements of the human will are reduced to some uniform cause, which, however, is above the intellect and will. This can be said, not of any body, but of some superior immaterial substance. Therefore there is no need for the movement of the will to be referred to the movement of the heavens, as to its cause.
Reply to Objection 2. The movements of the human body are reduced, as to their cause, to the movement of a heavenly body, in so far as the disposition suitable to a particular movement, is somewhat due to the influence of heavenly bodies; also, in so far as the sensitive appetite is stirred by the influence of heavenly bodies; and again, in so far as exterior bodies are moved in accordance with the movement of heavenly bodies, at whose presence, the will begins to will or not to will something; for instance, when the body is chilled, we begin to wish to make the fire. But this movement of the will is on the part of the object offered from without: not on the part of an inward instigation.
Reply to Objection 3. As stated above (Cf. I, 84, 6,7) the sensitive appetite is the act of a bodily organ. Wherefore there is no reason why man should not be prone to anger or concupiscence, or some like passion, by reason of the influence of heavenly bodies, just as by reason of his natural complexion. But the majority of men are led by the passions, which the wise alone resist. Consequently, in the majority of cases predictions about human acts, gathered from the observation of heavenly bodies, are fulfilled. Nevertheless, as Ptolemy says (Centiloquium v), "the wise man governs the stars"; which is a though to say that by resisting his passions, he opposes his will, which is free and nowise subject to the movement of the heavens, to such like effects of the heavenly bodies.
Or, as Augustine says (Gen. ad lit. ii, 15): "We must confess that when the truth is foretold by astrologers, this is due to some most hidden inspiration, to which the human mind is subject without knowing it. And since this is done in order to deceive man, it must be the work of the lying spirits."
Source: Summa Theologica (New Advent)