Inertial motion does not seem to require any cause. A thing put in motion persists in motion “by itself.” Hence, it seems that St. Thomas’ First Way of finding out what God is, if anything, is compromised. But not so fast, as we will see.
Is a moving thing always moving toward something? At first glance, this seems preposterous. A rock M careening through space is not moving toward anything; it’s just moving. Having arrived at any arbitrary location, it immediately abandons it and keeps on trucking. It does not seem to care where it is going. It does not plan or arrange its affairs in such a way as to make it more likely to end up in this point as opposed to that one.
However, this conclusion is premature. For it is the case that for any point X on its future trajectory, M is moving toward it. We can verify that by waiting a bit and observing M arrive to X; or by doing some calculations, we can predict that it would be at X in the future.
Anything that is in potency toward Q is actuated in its motion toward Q by something that is fully and 100% at Q. That M is on its way toward X must be ultimately due to something that has already arrived at X, has no interest in leaving X, and finally is wholly in X.
If the general principle is correct, then we may surmise that “the thing which is God” is omnipresent in every point in space (as 1st-level “divine” energy) that has ever been or ever will be visited by any material object. It is, moreover, wholly present in such points, and cannot be pushed out of them. But the latter point entails that God is unmoved and immovable.
The argument will consist of 3 steps. First, all motion is relative; absolute motion is undetectable by any created being. However, consider a 3D Cartesian coordinate system, with 3 billiard balls moving inertially at a constant speed from the origin along the 3 axes. There is no reference frame from whose point of view all 3 balls are stationary. In other words, for all reference frames, there exists detectable motion relative to that frame. In our universe in particular, it is obvious that there prevails a general phenomenon of motion, regardless of the reference frame, and even necessarily so, as part of the essence of space.
This is getting very close to the idea that there is such a thing as absolute motion, however hidden from our apprehension. But, and second, God can perceive absolute motion as explained earlier (i.e., via His spatial omnipresence).
That provides another reason for ascribing essential immovability to God, since otherwise God would be just another object and able to detect only relative (to Him) motion.
Finally, a material object M thereby moving absolutely from a definite point X to point Y, with God keeping an eye on it, has an act — location, and potency — momentum. It is at point X, but is in potentiality toward leaving it at once and moving closer (since motion is unequivocally shown to be discreet by the Zeno’s paradox) to Y, say, by visiting X’ first. Something which is already in actuality in regard to X’ must move M into it.
I’ll call the pure act thereby conceived, i.e., the unmoved mover, God.