Since the heavenly bodies are in constant motion, so far as we can judge, it may seem that if their substance remains, they will keep on moving also in the state of consummation. And, indeed, if motion were possessed by heavenly bodies for the same reason as that for which it is possessed by elements, such an assertion would be logical. Motion is found in heavy or light elements to promote the perfection they are to attain: by their natural motion they tend to the place that suits them, where they are in a better condition. Hence in the ultimate state of consummation each element and each part thereof will be in its own proper place.
But this cannot be maintained of the motion of heavenly bodies, for a heavenly body does not come to rest in any place it may occupy; as it travels naturally to any particular place, it no less naturally departs thence. Therefore heavenly bodies suffer no loss if they are deprived of motion, because motion is not found in them for their own perfection. Also, it would be ridiculous to contend that a heavenly body is moved in circles by its nature as an active principle, in the way that a light body is impelled upward by its nature. For, as is evident, nature tends invariably in the direction of unity; and therefore that which by its very concept opposes unity cannot be the ultimate goal of nature. But motion is opposed to unity, in the sense that what moves varies in its mode of being by the very fact that it is in motion. Therefore nature does not produce motion just for the sake of motion, but in causing motion has in view the terminus to be reached by motion.
For instance, a body that is naturally light seeks an elevated place in its ascent; and so of other bodies. Consequently, since the circular motion of a heavenly body does not tend to a definite position, we cannot say that the active principle of a heavenly body’s circular motion is nature, in the sense that nature is the principle of the motion of heavy and light bodies. Accordingly there is no reason why heavenly bodies should not come to rest, without any change in their nature, even though fire, if its nature is to remain constant, cannot cease from its restlessness as long as it exists outside its proper sphere. Nevertheless we say that the motion of a heavenly body is natural; but it is natural not by reason of an active principle of motion in it, but by reason of the mobile body itself that has an aptitude for such motion. We conclude, therefore, that motion is communicated to a heavenly body by some intellect.
However, since an intellect does not impart movement except in view of some end, we must inquire what is the end of the motion of heavenly bodies. The motion itself cannot be said to be this end. For motion is the way leading to perfection, and so does not verify the concept of end, but rather pertains to that which is tending toward an end. Likewise we cannot maintain that a succession of locations is the term of the movement of a heavenly body, as though a heavenly body moved for the purpose of actually occupying every position for which it has a potency; this would entail endless wandering, and what is endless contradicts the notion of end.
We ought to think of the end of the heaven’s motion somewhat as follows. Any body set in motion by an intellect is evidently an instrument of the latter. But the end of an instrument’s motion is a form conceived by the principal agent, a form that is reduced to act by the motion of the instrument. The form conceived by the divine intellect, to be realized by the motion of the heavens, is the perfection of things. as achieved by way of generation and corruption. But the ultimate end of generation and corruption is the noblest of all forms, the human soul; and the soul’s ultimate end is eternal life, as we said above. Accordingly the ultimate end of the movement of the heavens is the multiplication of men, who are to be brought into being for eternal life.
Such a multitude cannot be infinite; the intention to be realized by any intellect comes to rest in something definite. Consequently, once the number of men who are to be brought into being for eternal life is filled out, and they are actually established in the possession of eternal life, the movement of the heavens will cease, just as the motion of any instrument ceases after a project has been carried through to completion. And when the movement of the heavens ceases, all movement in lower bodies will cease by way of consequence, excepting only the movement that will be in men as flowing from their souls. And thus the entire material universe will have a different arrangement and form, in accordance with the truth proclaimed in 1 Corinthians 7:31: “The shape of this world passes away.”
Thomas Aquinas, Compendium Theologiae, 171