By Daniel Shields

The following essay is a response to the review “Recovering Aquinas’s Motion Proof.” This post has been updated.

I am immensely grateful that John Brungardt has taken the time to read my book carefully, and I am greatly encouraged that he finds it helpful and is overall in agreement with it. The following is a friendly response to some of his criticisms.

My first point concerns the doubly disjunctive character of G2 in the Summa contra Gentiles. Aquinas does consider the possibility that motion in the cosmos is caused by “sphere-souls,” but Brungardt left out of his summary what I considered to be a key-point in chapter four: the “sphere-souls” that Aquinas considers are not souls at all according to Aquinas. They are, in fact, wholly immaterial beings that are united to the celestial spheres they move only as movers, not as substantial forms. The only sense in which they are called-improperly-“souls” is that their action is limited to one place, to the sphere they move, and they cannot cause motion anywhere else. Nor does Aquinas mean by this that the sphere-soul could not possibly act elsewhere, but only that it cannot act in more than one place at once. This shows, in his view, that such an immaterial being must have its action coordinated with that of others by a higher being whose sphere of influence is not limited to a single place. Whether we follow Aquinas in holding that such a coordinating being is necessary, the motion proof has still shown us that one or more fully immaterial beings that are ultimately responsible for motion in the cosmos exist. Thus the motion proof has already proved the existence of one or more immaterial gods before the Fifth Way is even utilized.

Something that is also worth pointing out is that I argue in the book that the Third Way does the same duty in ST as G2 does in SCG, so that one can extend the First Way entirely from within the confines of ST I, q. 2, a. 3. My thinking about this has developed a little since writing the book. At the time I still thought of the Third Way as a more metaphysical argument, but I now see it more as a natural philosophical argument as well, at least its first part. The First Way is extended by the Third Way to a more robust conclusion: one or more self-necessary beings exist. It is a short step from there to such beings’ immateriality. The Second Way can be seen as a necessary extension of the First Way for the sake of the Third Way: the second half of the Third Way is a straight-forward application of the Second Way to beings necessary through another. (I still don’t know how to interpret and defend the Fourth Way, but maybe someday I will present a coherent account of how all Five Ways fit together.)

Brungardt wonders whether the First Way might not imply—even if it does not directly argue—that a first mover must be pure act. Any body will be susceptible to change, and thus arguably not a first mover that terminates the regress of essentially ordered movers: “That is, the First Way as it stands seems rather to imply—by relying upon the act-potency argument for the mover principle—that any movers susceptible to change will themselves fall under the dependency relationship highlighted in the first premise. The intensional scope of the First Way seems to suffice to lead mediately to the conclusion that the first mover is pure act.”

But just because a mover is undergoing motion as its moves another body does not mean that it cannot be the first unmoved mover that terminates the essentially ordered series of movers. I’m sure that sounds strange to most Thomists, but it is what follows from the logic of Aquinas’ argument. The impossibility of infinite regress is due to the fact that one needs a non-instrumental mover, a mover that itself possesses the wherewithal to cause the motion in question. A body could be such a first, non-instrumental mover for another body, while at the same time undergoing some other motion irrelevant to its ability to cause the motion in question. In fact, due to Newton’s 3rd law—that for every action there is an equal and opposite reaction—this will always be the case when one body moves another. They move each other concurrently, with two separate motions, and each can be a first unmoved mover of the other. I argued this in my book, but since then Schmid and Linford have made the same argument against the First Way as normally interpreted in chapter one of their book Existential Inertia and Classical Theistic Proofs. No Thomist has yet to respond to them adequately, but my interpretation of the First Way entirely sidesteps their criticism.

However, Brungardt has made a good point in his review: if one expands one’s consideration of the physical interaction of two bodies to consider the issue of free energy and entropy, one can say that no two bodies would be in a position to interact in the first place unless some source of free energy outside the universe had placed them in a position to do so. This means that the First Way, with a slight extension, could get one to an immaterial mover. (I say this tentatively, for I need to give the issue further thought.) But that does not immediately mean that this immaterial mover is pure act. For all the argument shows, it could have all kinds of objective potencies for acts irrelevant to its ability to move the bodies in question. Furthermore, for all the argument has shown, there might be several such unmoved movers moving different parts of the universe. (The Fifth Way helps with this issue.)

Brungardt objects to my realism about inertia, but I’m not sure to what extent our disagreement is substantive and to what extent it is verbal. Brungardt is willing to speak of inertia as having an “abstract-and-yet-real status.” My approach to inertia is based on a general methodological principle: I take the claims of science as true and real unless I have a very good reason not to. So I default to treating inertia as a real principle of bodily motion. Now inertia has never meant that bodies move in uniform rectilinear motion except when acted on by an external force. Newton says in his first law that bodies move in uniform motion except to the extent that (nisi quatenus) they are acted on by an external force. Every body I have had experience with does this. When I throw things they tend to continue in motion after leaving my hand and they tend to go in straight lines. A good example is when a stone is released from a sling: it clearly tends to move off in a line tangent to the circle of the sling at the point it is released from. To get an object to move in any curve other than one caused by gravity I need to give it a spin, and I find this hard to do. (I am a terrible pitcher.) The tendency of a body to move downwards is also obvious to sense-experience, and the path of any projectile is fairly obviously compounded of a rectilinear inertial component and a downwards gravitational component.

This is how things look at first glance to the senses, but deeper experience and analysis shows—through Einstein’s general relativity—that gravity and inertia are a single tendency, not two. And thus overall objects naturally move in giant conic sections, curved paths that are very nearly but not quite straight. I accept the general relativistic account, but this account is compatible with the historical Thomistic account that holds that the natural gravitational motion of a body is due to a natural active principle within it. I do not think that the gravitational field pushes the body around; it merely provides a physical, contacting medium by which a body can have its natural active principle of motion determined to a certain direction and strength. It is the body’s nature to head towards the location in the field of lowest gravitational potential energy.

The telos of inertial motion is complete isolation of a body from all others. Contemporary cosmology holds that the expansion of the universe is accelerating. This means that a body moving inertially between galaxies could eventually find itself so far from every other body in the universe that the continual expansion of the space between it and other bodies renders it impossible for light, gravitational influence, or any other causal influence to ever be able to reach it, since such causal influences cannot travel faster than the speed of light. No part of space expands faster than the speed of light, but all the parts of space collectively do, and thus causal influences can never traverse the space. A body moving inertially thus eventually reaches a condition of total isolation, and then locomotion means nothing for it, since locomotion always involves relationships between bodies. It is effectively at rest. Inertial motion can achieve this telos.

But I have been speaking again as if inertia is distinct from gravitational motion, whereas in reality it is just gravitational motion looked at from a different frame of reference. Thus inertia has two possible tele: either total isolation of the kind described, or collapse with other bodies into a black hole. (These are just two versions of the same telos: a kind of cessation of interaction with other bodies, total thermal equilibrium.) In either case, the gravitational field that constitutes space leads eventually to the end of motion, unless God is at work in the universe.

Brungardt raises the possibility that an unmoved mover argument might deliver a more robust conclusion if the arguments from Physics VII.1 are taken into account. He may be right, and I welcome any attempt to make sense of these arguments. Nothing I have read so far has shed much light on them, and after sustained reflection I still cannot make any sense of them myself. I cannot evaluate Brungardt’s attractive suggestions until these arguments are clarified for me. Thus my own account of the First Way is tentative, since I admittedly have not put every piece of the puzzle into place.