Objection 1. It would seem that the heavenly bodies are the cause of human actions. For since the heavenly bodies are moved by spiritual substances, as stated above (Question 110, Article 3), they act by virtue thereof as their instruments. But those spiritual substances are superior to our souls. Therefore it seems that they can cause impressions on our souls, and thereby cause human actions.
Objection 2. Further, every multiform is reducible to a uniform principle. But human actions are various and multiform. Therefore it seems that they are reducible to the uniform movements of heavenly bodies, as to their principles.
Objection 3. Further, astrologers often foretell the truth concerning the outcome of wars, and other human actions, of which the intellect and will are the principles. But they could not do this by means of the heavenly bodies, unless these were the cause of human actions. Therefore the heavenly bodies are the cause of human actions.
On the contrary, Damascene says (De Fide Orth. ii, 7) that "the heavenly bodies are by no means the cause of human actions."
I answer that, The heavenly bodies can directly and of themselves act on bodies, as stated above (Article 3). They can act directly indeed on those powers of the soul which are the acts of corporeal organs, but accidentally: because the acts of such powers must needs be hindered by obstacles in the organs; thus an eye when disturbed cannot see well. Wherefore if the intellect and will were powers affixed to corporeal organs, as some maintained, holding that intellect does not differ from sense; it would follow of necessity that the heavenly bodies are the cause of human choice and action. It would also follow that man is led by natural instinct to his actions, just as other animals, in which there are powers other than those which are affixed to corporeal organs: for whatever is done here below in virtue of the action of heavenly bodies, is done naturally. It would therefore follow that man has no free-will, and that he would have determinate actions, like other natural things. All of which is manifestly false, and contrary to human habit. It must be observed, however, that indirectly and accidentally, the impressions of heavenly bodies can reach the intellect and will, forasmuch, namely, as both intellect and will receive something from the inferior powers which are affixed to corporeal organs. But in this the intellect and will are differently situated. For the intellect, of necessity, receives from the inferior apprehensive powers: wherefore if the imaginative, cogitative, or memorative powers be disturbed, the action of the intellect is, of necessity, disturbed also. The will, on the contrary, does not, of necessity, follow the inclination of the inferior appetite; for although the passions in the irascible and concupiscible have a certain force in inclining the will; nevertheless the will retains the power of following the passions or repressing them. Therefore the impressions of the heavenly bodies, by virtue of which the inferior powers can be changed, has less influence on the will, which is the proximate cause of human actions, than on the intellect.
To maintain therefore that heavenly bodies are the cause of human actions is proper to those who hold that intellect does not differ from sense. Wherefore some of these said that "such is the will of men, as is the day which the father of men and of gods brings on" (Odyssey xviii 135). Since, therefore, it is manifest that intellect and will are not acts of corporeal organs, it is impossible that heavenly bodies be the cause of human actions.
Reply to Objection 1. The spiritual substances, that move the heavenly bodies, do indeed act on corporeal things by means of the heavenly bodies; but they act immediately on the human intellect by enlightening it. On the other hand, they cannot compel the will, as stated above (Question 111, Article 2).
Reply to Objection 2. Just as the multiformity of corporeal movements is reducible to the uniformity of the heavenly movement as to its cause: so the multiformity of actions proceeding from the intellect and the will is reduced to a uniform principle which is the Divine intellect and will.
Reply to Objection 3. The majority of men follow their passions, which are movements of the sensitive appetite, in which movements of the heavenly bodies can cooperate: but few are wise enough to resist these passions. Consequently astrologers are able to foretell the truth in the majority of cases, especially in a general way. But not in particular cases; for nothing prevents man resisting his passions by his free-will. Wherefore the astrologers themselves are wont to say that "the wise man is stronger than the stars" [Ptolemy, Centiloquium, prop. 5], forasmuch as, to wit, he conquers his passions.
Source: Summa Theologica (New Advent)