Book One — God, Chapter 20 — Summa Contra Gentiles
[1] From the preceding remarks it is also shown that God is not a body.
[2] Every body, being a continuum, is composite and has parts. But, as we have shown, God is not composite, and is, therefore, not a body.
[3] Again, everything possessed of quantity is in a certain manner in potency. For a continuum is potentially divisible to infinity, while numbers can be increased to infinity. But every body has quantity and is therefore in potency. But God is not in potency, being pure act, as has been shown. Therefore, God is not a body.
[4] Furthermore, if God is a body, He must be some natural body, since, as the Philosopher proves, a mathematical body is not something self-existing, since dimensions are accidents. But God is not a natural body, being immobile, as we have shown, whereas every natural body is movable. God is, therefore, not a body.
[5] Again, every body is finite, as is proved in De caelo I [I, 5] of a circular body and a rectilinear body. Now, we can transcend any given finite body by means of the intellect and the imagination. If, then, God is a body, our intellect and imagination can think of something greater than God. God is thus not greater than our intellect—which is awkward. God is, therefore, not a body.
[6] Intellectual knowledge, moreover, is more certain than sensitive knowledge. In nature we find an object for the sense and therefore for the intellect as well. But the order and distinction of powers is according to the order of objects. Therefore, above all sensible things there is something intelligible among things. Now, every body having actual existence is sensible. Therefore, we can find something nobler above all bodies. Hence, if God is a body, He will not be the first and greatest being.
[7] A living thing, likewise, is nobler than any non-living body, and the life of a living body is nobler than it, since it is this life that gives to the living body its nobility above other bodies. Therefore, that than which nothing is nobler is not a body. Ibis is God. God is, therefore, not a body.
[8] Then, too, there are the arguments of the philosophers to the same effect, based on the eternity of motion. They are as follows. In every everlasting motion, the first mover cannot be moved either through Himself or by accident, as is clear from the above. Now, the body of the heavens is moved in a circle with an everlasting motion. Therefore, its first mover is not moved either through Himself or by accident. Now, no body moves locally unless it be moved, since the mover and the moved must be together. The moving body must thus be moved in order to be together with the moved body. But no power in a body moves unless it itself be moved by accident, since, when a body is moved, its power is by accident moved. The first mover of the heavens, therefore, is neither a body nor a power in a body. Now, that to which the motion of the heavens is ultimately reduced as to its first unmoved mover is God. God is, therefore, not a body.
[9] Again, no infinite power is a power in a magnitude. But the power of the prime mover is an infinite power. Therefore, it is not in any magnitude. Therefore, God, Who is the prime mover, is neither a body nor a power in a body.
[10] The first proposition is proved thus. If the power of some magnitude is infinite, it will be the power either of a finite magnitude or an infinite one. But there is no infinite magnitude, as is proved in Physics III [5] and De caelo I [5]. But a finite magnitude cannot have an infinite power. Therefore, an infinite power cannot reside in any magnitude. That an infinite power cannot reside in a finite magnitude is proved thus. A greater power produces an equal effect in a shorter time than a lesser power does in a longer time. This is true whether that effect be according to alteration, local motion, or any other motion whatever. But an infinite power is greater than every finite power. Therefore, by moving more swiftly, it should produce its effect in a shorter time than any finite power. Nor can it be in something lesser that still is in time, Therefore, this will be in an indivisible point of time. And thus to move, to be moved, and motion will take place in an instant—of which the contrary has been proved in Physics VI [3]. That an infinite power in a finite magnitude cannot move in time is likewise proved as follows. Let there be an infinite power A. Let us assume a part of that power to be AB. This part will, therefore, move in a greater time. Yet there must be some proportion of this time to the time in which the whole power moves, since both times are finite. Let these two times be related to one another in the proportion of one to ten, since for the present argument this proportion will do as well as any other. Now, if we add to the aforementioned finite power, we must diminish its time according to the proportion of the addition to the power; for a greater power moves in a lesser time. If the decuple be added, that power will move in a time that will be a tenth part of the time in which the first assumed part of the infinite power, namely, AB, moved. And yet this power, which is its decuple, is a finite power, since it has a determinate proportion to the finite power. Therefore, the finite and the infinite power will move in the same time—which is impossible. Therefore, the infinite power of a finite magnitude cannot move in time.