Thomas Aquinas, Compendium of Theology

Everything that is changeable and multiform is traced back, as to its cause, to some first principle that is immobile and is one. Since man's intellect and will are clearly changeable and multiform, they must be reduced to some higher cause that is immobile and uniform. The heavenly bodies are not the cause to which they are reduced, as we have shown; therefore they must be reduced to yet higher causes.

In this matter the case of the intellect differs from that of the will. The act of the intellect is brought about by the presence of the things understood in the intellect; but the act of the will is accounted for by the inclination of the will toward the things willed. Thus the intellect is adapted by its nature to be perfected by something external that is related to it as act to potency. Hence man can be aided to elicit an act of the intellect by anything external that is more perfect in intelligible being: not only by God but also by an angel or even by a man who is better informed; but differently in each instance. A man is helped to understand by a man when one of them proposes to the other an intelligible object not previously contemplated; but not in such a way that the light of the intellect of one man is perfected by the other, because each of these natural lights is in one and the same species.

But the natural light of an angel is by nature of a higher excellence than the natural light of man, and so an angel can aid a man to understand, not only on the part of the object proposed to him by the angel, but also on the part of the light that is strengthened by the angel's light. However, man's natural light does not come from an angel, for the nature of the rational soul, which receives existence through creation, is produced by God alone.

God helps man to understand, not only on the part of the object proposed by God to man, or by an increase of light, but also by the very fact that man's natural light, which is what makes him intellectual, is from God. Moreover, God Himself is the first truth from which all other truth has its certitude, just as secondary propositions in demonstrative sciences derive their certitude from primary propositions. For this reason nothing can become certain for the intellect except through God's influence, just as conclusions do not achieve certitude in science except in virtue of primary principles.

With regard to the will, its act is a certain impulse flowing from the interior to the exterior, and has much in common with natural tendencies. Accordingly, as natural tendencies are placed in natural things exclusively by the cause of their nature, the act of the will is from God alone, for He alone is the cause of a rational nature endowed with will. Therefore, if God moves man's will, this is evidently not opposed to freedom of choice, just as God's activity in natural things is not contrary to their nature. Both the natural inclination and the voluntary inclination are from God; each of them issues in action according to the condition of the thing to which it pertains. God moves things in a way that is consonant with their nature.

This exposition brings out the fact that heavenly bodies can exert an influence on the human body and its bodily powers, as they can in the case of other bodies. But they cannot do the same with regard to the intellect, although an intellectual creature can. And God alone can touch the will.