Today we start the second part of Book Three. Only another year and a half or so to go in Book Three. Then, finally, Book four! Our intellects are not material, but our bodies are. Do read all arguments, and especially think of the last.
1 From the things set forth earlier it is immediately evident that celestial bodies cannot be causes of events which go on in the understanding. Indeed, we have already shown that the order of divine providence requires the lower things to be ruled and moved by the higher ones. But the understanding surpasses all bodies in the order of nature, as is also clear from what we have said before. So, it is impossible for celestial bodies to act directly on the intellect. Therefore, they cannot be the direct cause of things that pertain to understanding.
Notes It’s not only that stars can’t control your intellect; neither can things terrestrial. See argument 3 below. Also, astrology (of a sort) is not dismissed. Again, read through to the end.
2 Again, no body acts except through motion, as is proved in Physics VIII . But things that are immovable are not caused by motion, for nothing is caused by the motion of an agent, unless the agent moves a passive subject during the motion. So, things that are utterly apart from motion cannot be caused by the celestial bodies. But things that are in the area of understanding are entirely apart from motion, properly speaking, as is evident from the Philosopher, in Physics VII . On the contrary, “through being undisturbed by motions, the soul becomes prudent and knowing” as is stated in the same place. Therefore, it is impossible for celestial bodies to be the direct cause of things that pertain to understanding
3 Besides, if nothing is caused by a body unless the body is moved while the motion is going on, it is necessary for everything that receives an impression from a body to be moved. Now, nothing is so moved except a body, as is proved in Physics VI . So, everything that receives an impression from a body must be a body, or some power of a body. Now, we showed in Book Two that the intellect is neither a body nor a bodily power. Therefore, it is impossible for the celestial bodies directly to make an impression on the intellect.
4 Moreover, everything that is moved by another thing is reduced by it from potency to act. But nothing is reduced by a thing from potency to act unless that thing is actual. So, every agent and mover must be in some way actual, in regard to the effects to which the passive and movable subject is in potency. Now, the celestial bodies are not actually intelligible, for they are certain individual, sensible things. And so, since our intellect is not in potency to anything except actual intelligibles, it is impossible for celestial substances directly to act on the intellect.
5 Furthermore, the proper operation of a thing depends on its nature, which, in things that are generated, is acquired, along with the proper operation, through the process of generation. This is clear in the case of heavy and light things, which immediately at the end of the process that generates them possess their proper motion unless there be some impediment. Because of this the generating agent is called a mover. So, that which in regard to the beginning of its nature is not subject to the actions of celestial bodies cannot be subject to them in regard to its operation. Now, man’s intellectual nature is not caused by any corporeal principles, but is of completely extrinsic origin, as we proved above. Therefore, the operation of the intellect does not come directly under the celestial bodies.
Notes The intellect is not material. Keep this always in mind.
6 Again, effects caused by celestial motions are subject to time, which is “the measure of the first celestial motion.” And so, events that abstract from time entirely are not subject to celestial motions. But the intellect in its operation does abstract from time, as it does also from place; in fact, it considers the universal which is abstracted from the here and now. Therefore, intellectual operation is not subject to celestial motions.
7 Besides, nothing acts beyond the capacity of its species. But the act of understanding transcends the species and form of every sort of bodily agent, since every corporeal form is material and individuated, whereas the act of understanding is specified by its object which is universal and immaterial. As a consequence, no body can understand through its corporeal form. Still less, then, can any body cause understanding in another being.
Notes The pun is that students must teach themselves.
8 Moreover, a being cannot be subject to its inferiors by the same part whereby it is united to its superiors. But our soul is united to the intellectual substances, which are superior to the celestial bodies in the order of nature, by virtue of the part which is the understanding. In fact, our soul cannot understand unless it receives intellectual light from those substances. Therefore, it is impossible for intellectual operation directly to be subject to the celestial motions.
9 Furthermore, our confidence in this view will be increased if we consider the statements of the philosophers on the point. As a matter of fact, the ancient natural philosophers, like Democritus, Empedocles, and those of similar persuasion, claimed that understanding does not differ from sense perception, as is evident from Metaphysics IV  and from Book III of On the Soul .
And so, the conclusion was made that, since sensation is a bodily power depending on changes in bodies, the same thing is also true of understanding. For this reason, they said that intellectual operation results from the motion of the celestial bodies, because change in lower bodies results from change in the higher bodies. According to a passage in Homer: “So understanding in gods and in earthly men is like the daylight which the father of men and gods brings down”; the reference is to the sun, or, better, to Jupiter, whom they called the highest god, understanding him to be the whole heavens, as is clear from Augustine in his City of God [IV, 11].
10 Next came the opinion of the Stoics, who said that intellectual knowledge is caused by the fact that the images of bodies are impressed on our minds, as a sort of mirror or as a page receives the letters imprinted on it without its doing anything; as Boethius reports in Book V of the Consolation. According to their view, it followed that intellectual notions are impressed on us chiefly by an impression from the celestial bodies. Hence, the Stoics were the ones who especially asserted that the life of man is directed by a fatal necessity.
However, this theory appeared false, as time went on, as Boethius says in the same place, for the understanding combines and separates, compares the highest things with the lowest, and knows universals and simple forms that are not found in bodies. So, it is obvious that the understanding is not simply receptive of bodily images, but has a power higher than bodies, since external sensation which is only receptive of bodily images does not encompass the actions mentioned above.
11 Now, all the philosophers who followed distinguished understanding from sense perception and attributed the cause of our knowledge not to bodies, but to immaterial things. Thus, Plato claimed that the cause of our knowledge is the Ideal Forms; while Aristotle said that it is the agent intellect.
12 From all these views we may gather that the assertion that the celestial bodies are the cause of our act of understanding is a consequence of the opinion of those who claimed that understanding does not differ from sensation, as is clear from Aristotle in his book On the Soul [III, 3]. Now, it has been shown that this opinion is false. So, it is also obvious that the opinion which asserts that celestial bodies are directly the cause of our act of understanding is false.
13 Hence, Sacred Scripture also ascribes the cause of our understanding, not to any body but to God: “Where is God, Who made me, Who gives songs in the night; Who teaches us more than the beasts of the earth, and instructs us more than the fowls of the air?” (Job 35:10-11). Again, in the Psalm (93:10): “He who teaches man knowledge.”
Notes After all, universals are infinite, and God is infinite, and to be moved to the infinite requires God.
14 However, we should note that, though celestial bodies cannot be directly the causes of our understanding, they may do something indirectly in regard to it. For, although the understanding is not a corporeal power, the operation of understanding cannot be accomplished in us without the operation of corporeal powers: that is, the imagination, the power of memory, and the cogitative power, as is evident from preceding explanations. And as a result, if the operations of these powers are blocked by some indisposition of the body, the operation of the intellect is impeded, as is evident in demented and sleeping persons, and in others similarly affected.
And that is why even the good disposition of the human body makes one able to understand well, for, as a result of this, the aforesaid powers are in a stronger condition. Thus it is stated in Book II of On the Soul  that we observe that “men with soft flesh are well endowed mentally.”
Now, the condition of the human body does come under the influence of celestial motions. In fact, Augustine says, in the City of God V, that “it is not utterly absurd to say that certain influences of the stars are able to produce differences in bodies only.” And Damascene says, in Book II [De fide orthodoxa], that “different planets establish in us diverse temperaments, habits and dispositions.” So, the celestial bodies work indirectly on the good condition of understanding.
Thus, just as physicians may judge the goodness of an intellect from the condition of its body, as from a proximate disposition, so also may an astronomer judge from the celestial motions, as the remote cause of such dispositions. In this way, then, it is possible that there is some truth in what Ptolemy says in his Centiloquium: “When, at the time of a man’s birth, Mercury is in conjunction with Saturn and is itself in a strong condition, it gives inwardly to things the goodness of understanding.”
Notes These days, of course, we are more apt to render the aphorism men with hard flesh are well endowed mentally, life having become too easy.