How can Aquinas' argument from motion to mover be reconciled with Newton's law of inertia?

Question

A common objection to Thomas Aquinas' first way, the argument from motion (which means rather something like change), is that the second premise is flawed:

It is certain, and evidence to our senses, that some things are in motion. Now whatever is in motion is put in motion by another, for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves inasmuch as it is in act. If that by which it is moved be itself moved, then this also must needs to be moved by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover, seeing that subsequent movers move only inasmuch as they are moved by the first mover: as the staff moves only because it is moved by the hand. Therefore it is necessary to arrive at a first mover, moved by no other; and this everyone understands to be God.”

(Summa Theologiae - First Part: Question 2, article 3)

The flaw in the premise comes supposedly from working with an outdated physics which does not incorporate Newton's Law of Inertia (= LI). In Newton's own words the LI is stated as:

Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon.

Philosophiae Naturalis Principia Mathematica, Law I.

A lot of replies are possible to this objection. Probably the most fascinating (and to me the most convincing) would be, that one modern possible explanation for inertia relies on the quantum fluctuations of empty space. So this would be the “mover” in this case.

Anyway, if we reject the idea that spatial movement is not a kind of change or movement in the Thomistic-Aristotelian sense – which I think we definitely should –, we must find a “mover”, or, in my view, the premise is seriously put into question.

I'm not completely unsympathetic to the idea that a strong and clear-cut contradiction can put a straight-forward interpretation of an empirical finding (in this case: “There is no ‘mover’!”) into question. This is because any straight-forward or minimal interpretation of an empirical finding appeals to Occam's razor, which is itself just a principle of reason.

Still, I don't see such a contradiction in the LI or all three Newtonian axioms. Neither do I see it in the metaphysical theory that infinite change can be brought about by a finite unchanging “mover”, like the momentum which a body possesses.

The LI may not be strictly true or may be incomplete and Aquinas may then have become vindicated, but for me this would seem to be just a highly contingent outcome. So, how can we justify the suspicion that in the case of the LI we just haven't found the “mover” yet? And why should we reject the metaphysical theory that infinite change can be brought about by a finite unchanging “mover”?

PS: Yes, this question is similar on the surface. But there the premise was probed in a more abstract way. Here we put the focus on the most serious concrete counterexample.

Answer 1

I would suggest instead that in the case of rectilinear uniform motion, for a single body, we accept that it is not a change or movement in the Thomistic-Aristotelian sense. Let me be careful here. Of course, this directly contradicts Aristotelian physics itself, but we are trying to adapt the doctrine to the world that accepts Newtonian mechanics, and relativity with quantum physics on top of it. In this world we need to distinguish physical change from the pseudo-change that is entirely due to our descriptive devices. I submit that rectilinear uniform motion belongs to that latter category (and arguably so do quantum mechanical collapse, and hole transformations in relativistic gauge theories). Aristotle is unlikely to have accepted looking at an apple from a different angle as a change in the apple. If he learned of the nominal character of the rectilinear uniform motion he is more likely to have discarded his physics rather than his metaphysics, and adopted the same position.

A sign that rectilinear uniform motion is not a physical process is in the fact that it can be eliminated by choosing an appropriate reference frame, namely the one comoving with the body. This would not work with other types of motion because comoving frames introduce real physical effects, the so-called fictitious forces like the centrifugal, which are real enough to kill you. But none of that happens under the rectilinear uniform motion, all inertial frames are physically equivalent in Newtonian physics as well as in relativity. If we analogize spacetime to space "moving" uniformly amounts to choosing coordinate axes. The only "change" is that of conventions and spatiotemporal location. To make it "real" we must take the spacetime per se to be real, and indications are that such spacetime substantivalism goes against the grain of modern physics. Einstein's hole argument is directed against it:

"If one has two distributions of metric and matter fields related by a hole transformation, manifold substantivalists must maintain that the two systems represent two distinct physical systems. This physical distinctness transcends both observation and the determining power of the theory since: The two distributions are observationally identical. The laws of the theory cannot pick between the two developments of the fields into the hole."

And here is Einstein's conclusion in his own words:

"Formerly, people thought that if matter disappeared from the universe, space and time would remain. Relativity declares that space and time would disappear with matter."

Fictitious change requires no mover. But while rectilinear uniform motion of a single body is physically indistinguishable from rest, rectilinear uniform motion of two bodies relative to each other is physical: there is no frame in which both are at rest. One way to deal with it is to "meta" the above reasoning. In Aristotelian physics rest is the only "baseline" state, everything else requires a mover, but this is not the case in Newtonian, let alone modern physics. They admit multiple "baseline" states known as vacua. Being in a vacuum state requires no "cause", only exciting out of it does. Nonetheless the so-called zero-point energy of vacua is positive, which means that something is going on there (in quantum field theory it is sometimes identified with incessant creation/annihilation of virtual particles). In our case we can declare systems where all bodies move rectilinearly and uniformly relative to each other to be mechanical vacua, on equal footing with the Aristotelian total rest. It would be a stretch to call the steady-state change in vacua "fictitious", but in a way it is uninteresting, vacuous, the motto then becomes that vacuous change requires no cause. It is in line with the Newtonian physics where the cause, force, is responsible for only for accelerating bodies, not just moving them.

If this is deemed unsatisfactory, we should recall that Aquinas's understanding of causes is more subtle than of those in temporal chains of events. If we do only admit the total rest as baseline it is natural to inquire how the bodies acquired those uniform velocities they display. The only way to get them starting from rest is to accelerate, which only forces ("movers") can do. Hence we still have a "mover", albeit a remote one. This still leaves the puzzle of the motion continuing after the mover stopped acting, but a solution to that was suggested as early as Philoponus, if not already Hipparchus, see Avempace, Projectile Motion, and Impetus Theory by Franco. It was that the mover impresses a force upon the body, which continues to move it even after the end of direct contact. It was needed to fix Aristotle's "theory" of projectile motion, which Philoponus mocked by pointing out that on it one could make an arrow fly by waving hands behind it. The impressed force, later dubbed impetus, was popular with Islamic Aristotelians, and with Buridan et al. in Europe soon after Aquinas. Mechanical momentum is the modern descendant of impetus, and the momentum conservation law can be interpreted as saying that changing momentum requires an intervention (external "mover"), while momentum itself is the internal "mover", the faint trace of past impressions.

Answer 2

There is no flaw. The Aristotelian concept of "motion" implies in Newtonian terms that whatever is being moved, is moved from rest at point A to rest at point B, and thus that there are non-zero net forces acting on the object: it's pushed forward and accelerated at first, and then being held back and decelerated later on, all that by objects other than itself.

That's because, for Aristotle, "motion" always refers to finite motion from somewhere and towards somewhere. He didn't contemplate the possibility of motion either from or towards infinity, which is what nonetheless better corresponds to Newton's concept of an unimpeded object in inertial motion.

Also, though it doesn't bear on the question, St. Thomas considers God and thus the prime mover to be infinite (see Question 7, Article I).

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Aristotle, Physics, Book VI, 241b8-13, translated by R. P. Hardie and R. K. Gaye

If, then, that which is in locomotion is to be changing to something, it must be capable of having changed [completing locomotion]. Consequently it's motion is not infinite, and it will not be in locomotion over an infinite distance; for it cannot have traversed such a distance.

It is evident, then, that a process of change cannot be infinite in the sense that it is not defined by limits. [emphasis mine]

Answer 3

TLDR;

St. Thomas argues that a chain of causes of motion relates all the way back to the first cause. Drawing an analogy from the applied philosophy of law, in the context of legal reasoning one does not consider the remote, speculative, or ultimate source of cause. One deliberately cuts off the infinite chain of causes to recognize the proximate cause of events. Galileo and Newton do not speculate about the ultimate source of cause concerning a body in motion. Deriving his models in part from Galileo, Newton describes an ideal (counterfactual) model in which there are no sources of force acting on a body and therefore it would not change momentum, no matter how we specify the value, assuming the use of an inertial frame of reference.

Caveat on Fictitious Forces and Galilean Invariance

Fictitious forces, which are legitimate yet confusing, arise due to the possibility for imposing a non-inertial (accelerated) frame of reference:

https://physics.stackexchange.com/questions/55297/fictitious-forces-confusion

Fictitious motion is not a term discussed in physics. There is the description of arbitrary choice of inertial frame of reference which is well known as Galilean Relativity, or Galilean Invariance, or Galilean Transformation.

Counterfactual Models vs The Ultimate Cause

In the context of this question, relating to the ultimate cause of motion, human observers experience perceptions of motion; perceptions of causes; and the concept of an ultimate source of cause:

1. perceptions of motion;
2. perceptions of proximate cause; and
3. concepts of the ultimate source of cause.

Galileo is the founder of modern science in part because he used the periodic motion of a pendulum to specify uniform intervals of time and he used normalized units of length to specify the change in position of a moving body during each interval of time.

Galileo rolled smooth balls down a smooth inclined channel and across an elevated horizontal table. I suspect he then also allowed the balls to fall onto the floor of his laboratory. This three stage experiment informed his descriptions of accelerated downward motion, constant horizontal speed, and motion of a projectile in the form of a parabolic curve.

Galileo defines speed, which we call average speed, as the change in position (displacement) over the change in time (interval). Galileo defines accelerated motion, or acceleration, as the change in speed over the time interval. Galileo found that vertical motion is accelerated motion and horizontal motion occurs at constant speed for several intervals of time. Galileo states two ideal (counterfactual) physical laws as follows. In the absence of air resistance all bodies in downward vertical motion (ideal free fall) would fall at the same rate of acceleration. In the absence of resistance, a body projected at constant speed along an infinite horizontal plane would continue to move at constant speed.

In the thought experiment known as Galileo's Ship a human observer exists in the closed cabin of a ship sailing in a straight line on a perfectly smooth sea. Galileo argues that this observer, unable to look out the window or port hole, cannot tell by performing any experiments of motion whether the ship is moving with constant speed or sitting in dry dock on the shore. The reference frame in this thought experiment is what we now call an inertial reference frame.

Newton describes linear momentum as mass times linear velocity. He states that momentum is constant for a body unless it is acted upon by an external force. This generalizes Galileo's model for a body moving in a straight line at constant speed in the absence of resistance along the ideal horizontal plane. Instead of the absence of resistance there is the absence of external force where resistance and gravity are two examples of external forces.

According to Newton if a body interacts with the surroundings in a physical process, then the momentum of the body may or may not change, where an interaction that tends to change the momentum of the body gives rise to our concept of force laws. We must take the sum of forces to determine the net change in momentum which may be zero.

Galileo and Newton do not speculate about the ultimate source of cause concerning a body in motion. Deriving his models in part from Galileo, Newton describes an ideal (counterfactual) model in which there are no sources of force acting on a body and therefore it would not change momentum, no matter how we specify the value, assuming the use of an inertial frame of reference.