In the 13th chapter of the 1st book of his Summa contra Gentes, St. Thomas begins his argument for the existence of God from the effect of motion by saying, “Everything that is moved is moved by another [quod omne motum movetur ab alio].” The modern mind, however, might immediately recoil at this claim and, if it finds it repugnant enough, may not proceed to the remainder of the argument. After all, everything moves. “Every thing is in constant motion relative to other things,” our critic explains. “There is no absolute reference frame. This is basic physics.”
Yes, it is basic physics. It is not, however, Aristotelian physics, which is the discourse-scheme in which St. Thomas must be addressed. The difference is that Aristotle is referring to actuality and potentiality, not simple local motion. Actuality precedes potentiality as a metaphysical, albeit not logical, principle. That which is merely potential can actuate neither some other potential nor itself. This being so, we are led to account for the actuating basis of all the potential things (i.e., things “in motion” from potentiality to actuality) which we see in the universe. Motion for Aristotle and St. Thomas, then, is not primarily kinetic or potential (in the Newtonian-Leibnizian) sense, but Aristotelian motion can take the form of kinetic motion observed by scientists.
As our critic notes, everything moves—yet, as he also notes, nothing moves relative to (or “owing to”) itself, but only relative to any- and everything else. The pervasive dependence of material objects on other things for their own metaphysical actuation (and, derivatively, kinetic motion), is precisely the point of the argument from motion. Einsteinian relativity makes it a property of the entire universe to be in contingent, relative flux in various frames of reference, and this pancosmic potentiality is exactly what the argument from motion is hones in on. This potential dependence holds for objects in spacetime, as well as for space and time themselves. Precisely because there is no absolute time in the universe and no absolute motion, while there is plainly change-in-time, it falls to the Unmoved Mover to be a transcendent cause of dependent motion. Since nothing in the universe can be the absolute source and measure of motion and change, there must be a source of motion outside the cosmos, and one which is subject neither to material local motion nor metaphysical potentiality. This Actus Purus is what, among other things, the Church means by God.
Hence, if we push through our initial scientific reaction to St. Thomas’ ancient pre-scientific claim that “Everything that is moved is moved by another,” we will see how profound the argument from motion is. Several paragraphs later, St. Thomas adds this claim:
In an ordered series of movers and things moved (this is a series in which one is moved by another according to an order) [In moventibus et motis ordinatis, quorum scilicet unum per ordinem ab alio movetur], it is necessarily the fact that, when the first mover is removed or ceases to move, no other mover will move or be moved. For the first mover is the cause of motion for all the others. But, if there are movers and things moved following an order to infinity, there will be no first mover, but all would be as intermediate movers. Therefore, none of the others will be able to be moved, and thus nothing in the world will be moved.
Let us note that ordinal motion does not exactly mean ‘serial’ or ‘step by step’ motion as we are inclined to think of it. Rather, it refers to the idea of, let us say, distributed simultaneous efficiency. The efficient causation in an ordered causal system is distributed simultaneously throughout the elements involved at every moment of change. It is comprised of an essentially ordered series of metaphysically interdependent sub-causes and hemi-effects in simultaneous temporal co-relation.
For example, when a boy splashes water by hitting the surface of a creek with a stick, his hand, the stick, and the disrupted water are all, so to speak, causally concurrent. There is a proper order, a determinate structure, of this event, which cannot happen without all the elements being in the right place at the right— namely, the same—time. Moreover, we must realize that the boy’s hand simultaneously depends on its attachment to his body, his body on its attachment to the earth, the earth on its place in the solar system, and so on. Everything in the cosmos must occur in an exact causal, albeit not temporal, order for the water to splash as it does. This is more or less what St. Thomas means by what happens in motis ordinatis. Since no element of an essentially ordered causal series can cause the whole series—as it is immediately “responsible” for only its causally adjacent partner—and since the entire series cannot account for tiself—as it is comprised of numerous contingent sub-pieces likewise in need of actuation—therefore, we must look to God as the origin of all such causal systems.