Chapter 23 — That the movement of the heaven is from an intellective principle

It can also be shown from the foregoing that the first principle, which causes the heavenly movement, is intellective.

For nothing that acts according to its own species intends a form higher than its own, since every agent intends its like. (On Generation and Corruption, ch. 7.) Now a heavenly body, insofar as it acts by its own movement, intends the ultimate form, which is the human intellect, which is higher than any corporeal form, as we have proved above. (Ch. 22.) Therefore, the heavenly body acts for generation not in respect of its own species, as the principal agent does, but in respect of the species of some higher intellectual agent, in relation to which the heavenly body is like an instrument in relation to a principal agent. Now the heaven acts to the effect of generation inasmuch as it is moved. Therefore, the heavenly body is moved by some intellectual substance.

Again. Whatever is in motion must be moved by another, as we proved above. (Bk. 1, ch. 13.) Therefore, the body of heaven is moved by another. Now this other is either wholly separated from it, or it is united to it, so that what is composed of heaven and its mover is said to move itself inasmuch as one part of it is moved, and the other is mover. If this be the case, since whatever moves itself is living and animate, (Bk. 1, ch. 97.) it follows that the heaven is animate. And not otherwise than with an intellectual soul: for it could not be animated with a nutritive soul (since in the heavenly body there is no generation or corruption), nor with a sensitive soul (since the heavenly body has no variety of organs). Therefore, it follows that it is moved by an intellective soul. If, on the other hand, it be moved by an extrinsic mover, this will be either corporeal or incorporeal. If it be corporeal, it does not cause movement without being moved, for no body moves unless it be moved, as was shown above. (Bk. 2, ch. 22.) Thus this too will require to be moved by another. But as it is not possible to have an infinite series of bodies, we must come to some incorporeal first mover. Now that which is utterly separated from a body must be intellectual, as we have shown. (Bk. 1, ch. 44.) Therefore, the movement of the heaven, which is first among corporeal beings, is caused by some intellectual substance.

Moreover. Heavy and light bodies are moved by their generator and by that which removes obstacles, as is proved in 8 Physics, (Physics 8.4.) for it is impossible that their form be mover and the matter moved, for nothing is moved except a body. Now as the elemental bodies are simple, and there is no composition in them except that of matter and form, so too are the heavenly bodies simple. Hence, if they be moved as heavy and light bodies, it follows that they are moved essentially by their generator and accidentally by that which removes an obstacle. But this is impossible, for these bodies cannot be generated, because there is no contrariety in them, and their movements cannot be hindered. Therefore, these bodies must be moved by things that cause movement by a power of apprehension, which power cannot be sensitive, as we have proved. Therefore, it must be an intellective power.

Further. If the principle of the heavenly movement be from nature alone without any kind of apprehension, it follows that it must be the form of the heavenly body, as is the case with the elements. For, although simple forms do not cause movement, they are principles of movements, since natural movements follow from them like all other natural properties. Now it is impossible that the heavenly movement follow the form of the heavenly body as its active principle. For thus the form is the principle of local movement: insofar as a particular body, in respect of its form, is due a particular place to which it is moved by virtue of its form tending to that place. Because the generator gives this form, it is said to be a mover. Even so it is due to fire, in respect of its form, to be in a higher place. Now one place is not more due to a heavenly body in respect of its form than another. Therefore, nature alone is not the principle of the heavenly movement. Consequently, the principle of its movement must be something that moves it by apprehension.

Again. Nature always tends to one thing: therefore, things that come from nature always come in the same way, unless they be hindered, which seldom happens. Therefore, that which is essentially deformed cannot possibly be an end towards which nature tends. Now movement is essentially such, for that which is moved, as such, is conditioned otherwise now and before. (Physics 5.1.) Consequently, nature cannot intend movement for its own sake. Therefore, it intends through movement to obtain rest which in relation to movement is as one to many, for a thing is at rest which is conditioned in the same way now as before. Accordingly, if the heavenly movement were from nature alone, it would be directed to some kind of rest. But the contrary is the case, for it is unceasing. Therefore, the movement of the heaven is not from nature as its active principle, but from an intelligent substance.

Also. In every movement that is of nature as its active principle, if approach to a particular term be natural, recession from that term must be unnatural and contrary to nature: thus a heavy body naturally seeks a lower place, and recedes from it unnaturally. Therefore, if the movement of the heaven were natural, since it naturally tends to the west, it would be contrary to nature for it to return from the west to the east. But this is impossible, for nothing in the heavenly movement is violent or unnatural. Consequently, it is impossible for nature to be the active principle of the heavenly movement. Therefore, its active principle is some apprehensive power, which must be an intelligence, as we have proved above. Therefore, the heavenly body is moved by an intelligent substance.

And yet we must not deny that the heavenly movement is natural. For a movement is said to be natural not only on account of its active principle, but also on account of its passive principle. This is evident in the generation of simple bodies, since such generation cannot be called natural in relation to the active principle. For a thing to be moved naturally by an active principle, it must have this active principle within itself, for nature is a principle of movement in a thing in which it is; (Physics 2.1.) but the active principle in the generation of a simple body is without. Therefore, it is not natural by reason of its active principle, but only by reason of its passive principle (namely, matter), in which there is a natural appetite for its natural form. Accordingly, the movement of the heavenly body is not natural as to its active principle, but voluntary and intellectual; but it is natural as to its passive principle, since a heavenly body has a natural inclination for that movement.

This is made clear if we consider the relation of a heavenly body to its place. For a thing is passive and moved according as it is in potency, and it is active and moves according as it is in a state of act. Now a heavenly body, considered in its substance and as in potency, is indifferent to any place, even as prime matter is indifferent to any form, as we have stated. (Ch. 22.) But it is otherwise with a heavy or light body, which, considered in itself, is not indifferent to any place, and has a definite place appointed to it by reason of its form. Therefore, the nature of heavy and light bodies is the active principle of their movements, while the nature of a heavenly body is the passive principle of its movement.

Consequently, we must not think that it is moved by violence, like heavy and light bodies, which are moved by us through our intelligence. For heavy and light bodies have a natural aptitude for a movement contrary to that with which they are moved by us, and so they are moved by us violently; but the movement of an animal’s body, whereby that body is moved by the soul, is not violent to that body as animated (although it is violent insofar as that body is something heavy). On the other hand, the heavenly bodies have no aptitude for a contrary movement, but only for that in which they are moved by an intelligent substance. Consequently, it is both voluntary, as regards its active principle, and natural, as to its passive principle.

That the heavenly movement is voluntary in respect of its active principle is not inconsistent with the fact that it is one and uniform, because the will is indifferent to many things and is not determined to any one. For just as nature is determined to one by its power, so is the will determined to one by its wisdom, by which the will is unerringly directed to one end.

It is evident from the foregoing that neither approach to any one place nor recession from it is contrary to nature. For this happens in the movement of heavy and light bodies for two reasons. First, because the intention of nature, in heavy and light bodies, is determined towards one place: therefore, just as the body naturally tends to it, so does it recede from it against nature. Second, because two movements, one of which approaches a given term and the other recedes from it, are contrary. If, however, we take not the last but a middle place in the movement of heavy and light bodies, both approach to it and recession from it are natural, because the whole movement comes under the intention of nature, and the movements are not contrary, but are one continuous movement. It is the same in the movement of heavenly bodies, because the intention of nature is not towards one determinate place, as we have said already. Moreover, the movement with which a body moved in a circle recedes from any given place is not contrary to the movement with which it approaches towards it, but is one continuous movement: so that any given point in the heavenly movement is like a middle point, and not like the term in a straight movement.

As long as we admit that the heavenly movement is caused by an intellectual substance, it makes any difference as to the present question whether the heavenly body be moved by an intellectual substance united to it, so as to be its soul, or by a separate substance: nor whether each heavenly body be moved by God immediately; or none, and each be moved by the intermediary of created intellectual substances; or only the first heavenly body by God immediately, and the others through the intermediary of created substances.