This looks like a lot, but it is not that difficult. Our saint moves us swiftly through each argument. There is much to learn from this, even though, as the man said, times change.
1 From the preceding we can also show that the prime motive principle of the heavens is something intellectual.
2 Nothing that acts in function of its own species intends a form higher than its own form, for every agent tends toward its like. Now, a celestial body, acting under its own motion, tends toward the ultimate form, which is the human intellect; and which is, in fact, higher than any bodily form, as is clear from the foregoing. Therefore, a celestial body does not act for a generation according to its own species as a principal agent, but according to the species of a higher intellectual agent, to which the celestial body is related as an instrument to a principal agent. Now, the heavens act for the purpose of generation in accord with the way in which they are moved. So, a celestial body is moved by some intellectual substance.
3 Again, everything that is moved must be moved by another being, as we proved earlier. Therefore, a celestial body is moved by something else. So, this other thing is either completely separated from it, or is united with it in the sense that the composite of the celestial body and the mover may be said to move itself, in so far as one of its parts is the mover and another part is the thing moved.
Now, if it works this way, since everything that moves itself is alive and animated, it would follow that the heavens are animated, and by no other soul than an intellectual one: not by a nutritive soul, for generation and corruption are not within its power; nor by a sensitive soul, for a celestial body has no diversity of organs. The conclusion is, then, that it is moved by an intellective soul.
On the other hand, if it is moved by an extrinsic mover, this latter will be either corporeal or incorporeal. Now, if it is corporeal, it will not move unless it is moved, for no body moves unless it is moved, as was evident previously. Therefore, it will also have to be moved by another. And since there should be no process to infinity in the order of bodies, we will have to come to an incorporeal first mover. Now, that which is utterly separate from body must be intellectual, as is evident from earlier considerations. Therefore, the motion of the heavens, that is of the first body, comes from an intellectual substance.
4 Besides, heavy and light bodies are moved by their generating agent, and by that which takes away any impediment to motion, as was proved in Physics VIII [4: 256a 1]. For it cannot be that the form in them is the mover, and the matter the thing moved, since nothing is moved unless it be a body. Now, just as the elemental bodies are simple and there is no composition in them, except of matter and form, so also are the celestial bodies simple. And so, if they are moved in the same way as heavy and light bodies, they must be moved directly by their generating agent, and accidentally by the agent which removes an impediment to motion. But this is impossible, for these bodies are not capable of generation: for they are not endowed with contrariety, and their motions cannot, be impeded. So, these bodies must be moved by movers that function through knowing; not through sensitive knowledge, as we showed, but through intellectual knowledge.
5 Moreover, if the principle of celestial motion is simply a nature lacking any type of apprehension, then the principle of celestial motion must be the form of a celestial body, just as is the case in the elements. For, although simple forms are not movers, they are nonetheless the principles of motions, since natural motions are resultant from them, as are all other natural properties. Now, it is impossible for celestial motion to result from the form of a celestial body, as from an active principle. A form is the principle of local motion in the same way that a certain place is proper to a body by virtue of its form; it is moved to this place by the force of its form tending to it, and, since the generating agent gives the form, it is said to be the mover. For instance, it is appropriate to fire, by virtue of its form, to be in a higher place. But one place is no more appropriate than another for a celestial body, according to its form. Therefore, the principle of celestial motion is not simply the nature of the body. So, the principle of its motion must be something that moves as a result of apprehension.
6 Furthermore, nature always tends toward one objective; hence, things which result from nature always occur in the same way, unless they are interfered with, and this happens to few of them. Indeed, that which has a deformity within its very definition cannot be an end to which a nature tends. Now, motion, by definition, is of this type, for whatever is moved, by virtue of that fact, is in a different condition before and after. So, it is impossible for a nature to tend toward motion for the sake of motion. Therefore, it tends through motion toward rest, and the latter is related to motion as one to many. Indeed, a thing at rest is one which is in the same condition before and after. If then, the motion of the heavens were simply from a nature, it would be ordered to some condition of rest. But the contrary of this is apparent, for celestial motion is continuous. Therefore, the motion of the heavens does not arise from a nature, as its active principle, but rather from an intelligent substance.
7 Again, for every motion that is from a nature, as an active principle, if its approach to something be natural, then its removal from that objective must be unnatural and against nature. Thus, a heavy thing naturally moves downward, but for it to move in the opposite direction is against nature. Therefore, if the motion of the heavens were natural, since it tends westward naturally, it would return to the east in the manner of a thing that recedes from the west by a motion against nature. Now, this is impossible. In celestial motion there is nothing violent and against nature. So, it is impossible for the active principle of celestial motion to be a nature. Therefore, its active principle is some apprehensive power, and through understanding, as is clear from what was said earlier. So, a celestial body is moved by an intellectual substance.
8 Yet we must not deny that celestial motion is natural. In fact, a motion is called natural, not simply because of its active principle, but also because of its passive one. This is exemplified in the generation of simple bodies. Indeed, this generation cannot be called natural by reason of the active principle, for that is moved naturally by an active principle, which has its active principle within it; “a nature is a principle of motion in that to which it belongs.” But the active principle in the generation of a simple body is outside. So, it is not natural by reason of the active principle, but only by reason of the passive principle, which is the matter in which the natural appetite for a natural form is present. And so, the motion of a celestial body, as far as its active principle is concerned, is not natural, but voluntary and intellectual; however, in relation to its passive principle, the motion is natural, for a celestial body has a natural aptitude for such motion.
9 This becomes clearly evident when we consider the relation of a celestial body to its location. A thing is acted on passively, and is moved, in so far as it is in potency; while it acts and moves, in so far as it is in act. Now, a celestial body, considered in its substance, is found to be indifferently related to every place, just as prime matter is to every form, as we said before.
Of course, it is a different situation in the case of a heavy or light body which, considered in its nature, is not indifferent to every place, but is determined by virtue of its form to a place of its own. So, the nature of a heavy or light body is the active principle of its motion, while the nature of a celestial body is the passive principle of its motion. Hence, no one should get the impression that the latter is moved violently, as is the case with heavy and light bodies that are moved by us through understanding. For there is present in heavy and light bodies a natural aptitude for motion contrary to that in which they are moved by us, and so they are moved by us through violence. However, the motion of an animated body, in which it is moved by a soul, is not violent for it as an animal, though it is violent for it as a heavy object. Celestial bodies have no aptitude for contrary motion, but only for that whereby they are moved by an intelligent substance. Consequently, it is at once voluntary, in relation to the active principle, and natural, in relation to the passive principle.
10 That the motion of the heavens is voluntary according to its active principle is not repugnant to the unity and uniformity of celestial motion because of the fact that the will is open to a plurality of actions and is not determined to one of them. In fact, just as a nature is determined to one objective by its power, so is the will determined to one objective by its wisdom, whereby the will is infallibly directed to one end.
11 It is also evident from the foregoing that in celestial motion neither the approach to a certain place, nor the regression from that place, is against nature. Such a thing does occur in the motion of heavy and light bodies for two reasons. First, because the natural tendency in heavy and light things is determined to one place; hence, just as such a body naturally tends to this place, so does it go against nature in receding from it. Second, because two motions, one approaching a term and the other receding from it, are contrary. But, if we take into consideration in this motion of heavy and light bodies, not the final place but an intermediate one, then just as an approach may naturally be made to it, so also may a recession be naturally made from it. For the whole motion comes under one natural tendency, and these motions are not contrary but one and continuous. So, too, is the situation in the motion of celestial bodies, for the tendency of their nature is not toward some determinate place, as has been said already. Also, the motion whereby a body moves in a circle, away from a point of reference, is not contrary to the motion whereby it approaches the point, but it is one continuous motion. Hence, each place in the motion of the heavens is like a middle point, and not like a terminal point in straight-line motion.
12 Nor does it make any difference, as far as our present purpose is concerned, whether a heavenly body is moved by a conjoined intellectual substance which is its soul, or by a separate substance; nor whether each celestial body is moved immediately by God, or whether none is so moved, because all are moved through intermediary, created, intellectual substances; nor whether the first body alone is immediately moved by God, and the others through the mediation of created substances—provided it is granted that celestial motion comes from intellectual substance.