Does the Law of Inertia Disprove the Argument from Motion?

The argument from motion for the existence of God dates originally to Aristotle.  It is also presented by St. Thomas Aquinas as the First Way in his famous “Five Ways” [quinque viae] to demonstrate the existence of God. He calls it “the most manifest way.”

The demonstration proceeds from the evident premise that “some things are in motion” or “there is motion” to the necessity of a first or unmoved mover, something that can be shown to have the minimal attributes of what we call God. As with all proofs of the existence of God, the argument from motion proves only the bare existence of such a being: what God is like is something that still needs to be developed, and Thomas spends hundreds and hundreds of pages of careful argument doing this.  “You can’t prove anything about something unless you prove everything about it as part of the same argument” is an absurd principle.

Now, some have argued that Newton’s First Law of Motion, also known as the Law of Inertia, serves as a counterexample to, and therefore a refutation of, the argument from motion.  I’m going to show you why this is not the case.

Newton’s First Law states

Every body perseveres in its state of being at rest or of moving uniformly straight forward except insofar as it is compelled to change its state by forces impressed.

[NOTE 1: I had to go get my copy of Newton’s Principa to find this.  If you try to find it online, you’ll get nothing but paraphrases, of which there seems to be no established “correct” form. It’s worth noting that what is taken to be one of the basic laws of physics isn’t even stated clearly by anyone, anywhere, in a definite form!]

[NOTE 2: It’s also worth remarking that this basic law of nature applies to literally nothing. At first it seems to be about everything: “every body.”  But it turns out to be about “every body which is not acted upon by an outside force” which is no body at all—something which Newton’s own law of universal gravitation is enough to show. Every body is always being acted upon by every other body.  So this law of nature states what would happen in a situation that never happens.]

Fair enough. We do get the gist of the law. Aristotle thought, very reasonably but incorrectly, that things have a natural tendency to slow down and stop, because that is what we all observe things to do on earth.  The law of inertia states that they don’t have a natural tendency to change their velocity.  The word change is going to be important.

Now, the argument against the argument from motion would go:

  1. The argument from motion uses as a premise “Everything in motion requires a constant action upon it by something else to keep it in motion.”
  2. The law of inertia states that physical bodies in motion do not require a constant action upon them to keep them in motion.
  3. The law of inertia is true.
  4. So by the law of inertia, one premise of the argument from motion is false.
  5. So the argument is unsound.

Here we need to use Thomas’ favorite word: distinguo, “I distinguish” or “I make a distinction.”  A general rule of logic is that one needs to be working with clear and distinct terms.  If a term is ambiguous or equivocal, then one is in danger of the fallacy of equivocation, of using the ambiguous term now in sense, now in another.  When you encounter an ambiguity, it is necessary to make a distinction, or as many distinctions as necessary.  A good general rule of logical method is: to unmake an ambiguity, make a distinction.

It turns out that “motion” as used in the law of inertia isn’t “motion” as used in the argument from motion.  The argument against the argument from motion turns out to be invalid because of an equivocation on the term “motion.”

Here’s what Thomas says

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. For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality.

Helpfully, he gives us definition of the term “motion,” namely, “the reduction of something from potentiality to actuality.”

For the ancients and mediaevals, the word “motion” is synonymous with our word “change.”  “Change of place” or “local motion” was only one very common type of motion, so common in fact that the word “motion” itself eventually came to mean just “local motion” or “change of place.”

So the word “motion” originally meant “change” but came to mean “local motion” or “change of location.”  Wouldn’t it be odd if change of location turned out not even to be a kind of change? This seems to be the what happened.  The most obvious example of “change” turned out not to be a case of change!

We need another distinction, a very important one, drawn by the 20th century philosopher Peter Geach, between real properties  and real changes on the one hand and what he called “Cambridge properties” and “Cambridge changes,” on the other. (Calling them “Cambridge properties” is really a very funny jab at the state of philosophy at Cambridge in Geach’s day, but I won’t get into that here.)  Helpfully, Edward Feser has a paragraph about Cambridge properties and Cambridge changes, which saves me the trouble of writing one myself:

Here, building on a distinction famously made by Peter Geach, we need to differentiate between real properties and mere “Cambridge properties.” For example, for Socrates to grow hair is a real change in him, the acquisition by him of a real property. But for Socrates to become shorter than Plato, not because Socrates’ height has changed but only because Plato has grown taller, is not a real change in Socrates but what Geach called a mere “Cambridge change,” and therefore involves the acquisition of a mere “Cambridge property.” The doctrine of divine simplicity does not entail that God has no accidental properties of any sort; He can have accidental Cambridge properties.

A “Cambridge change” is a change in a “Cambridge property”, which is a property something has not by virtue of having a property (!) but by being in relation to something else, such that a change in the other thing causes a change in the relation, and so a merely relative change in a thing, insofar as it is a-thing-in-relation-with-something-that-changed, but not in itself.  Hard to say, but fairly easy to understand. Feser’s example is a good one: Plato was a very tall man. But it is true that at one time “Plato is shorter than Socrates” was true and later “Plato is taller than Socrates” was true—even though Socrates may have been completely unchanged in height during this time. Socrates’ height is a real property of Socrates. Socrates’ being shorter than Plato is not a real property of Socrates.  It is neither a quantity nor a quality, but a relative.  Such examples could be multiplied indefinitely: “Socrates is to the left of Plato” could be true, and then “Socrates is to the right of Plato” could be, if Plato walks to Socrates’ other side. Again, this could be done without Socrates’ changing position at all.

Why is this important? Well, it turns out that change of place, or local motion, or just “motion” in the modern sense isn’t a case of real change, but only a relative change.  “Motion” is not a substantial entity that is conserved, as Descartes thought.  As it turns out, momentum is the substantial entity that is conserved, lurking in the neighborhood of velocity as p = mv.

I could get into a lengthy discussion of Newtonian physics, but I need not. I’m just going to fast forward to Einstein and relativity theory.  We don’t even have to dive deep into Einstein, either.  It is sufficient to note that Einstein demonstrated that whether or not something is in motion or not is relative to one’s observational frame of reference (hence, relativity).   In other words, there seems to be no truth of the matter about whether something is in local motion or not; it depends on one’s frame of reference and the relation of the thing in question to other things.  You cannot say “X is moving” but only “X is moving relative to Y” or “X is stationary relative to Y.”  It seems like “X is moving” can be said in our, but it is really a statement comparable to “X is taller.” “X is taller” is a nonsense statement, because taller is not a quality, but a relative, and there can be no relatives with multiple relata—at least two.  “X is taller than Y,” on the other hand, makes perfect sense.  “X is moving” seems to make sense, I think, because we almost always make use of a “default frame of reference,” must likely the earth and everything that is generally affixed to the earth and therefore stationary (although most of us are conceptually aware that the earth is or could be moving—if we take the sun or the stars as our fixed frame of reference).  If I see a bird flying, it is very nearly impossible not to analyze this in my perception as the bird being in motion—although conceptually, I should be able to understand that I could make the bird the fixed thing which is stationary, and the earth and everything else in the universe would then move around the bird in a complicated manner. This is impractical and perhaps impossible in practice—but this is exactly the upshot of Einstein’s theory.

What this shows is that motion in modern sense is not a real change but only a Cambridge change.  And this is the motion that the law of inertia is talking about.

But if local motion is only a Cambridge change then local motion isn’t real motion, as understood by Aristotle and Aquinas.  And since isn’t a real motion, it isn’t a counterexample to the principle that “any real motion requires an external agency to bring it about.”

The principle that a real change or real motion, that is, a transition from potentiality to actuality, requires an outside agency is not refuted by the law of inertia.  In fact, the law of inertia preserves the principle entirely, because it states that a change in local motion (that is, a change in velocity) requires an external agency to bring it about.  Change of place turns out to be a mere Cambridge change, but change in velocity is a real change (because it entails a change in momentum, since p = mv, and momentum is a real property that is conserved)—and so it requires that the body which changes its velocity be acted upon by an external force.

So let’s reformulate the arguments

  1. The argument from motion has the premise “Every real change requires an external agency to bring it about.”
  2. The law of inertia states local motion will remain unchanged unless changed by an external agency.
  3. The law of inertia is true.
  4. Local motion is a real change.
  5. So local motion is counterexample to the premise used in the argument from motion.
  6. So the argument from motion has a false premise.
  7. So the argument from motion is unsound.

This refutation of the argument from motion fails because premise 4 is false.  And since 4 is false, 5 is false. And since 4 and 5 are false, 6 and 7 are not demonstrated.

We can present the proper argument thus:

  1. The argument from motion has the premise “Every real change requires an external agency to bring it about.”
  2. The law of inertia states that change of place of bodies does not require an external agency to bring it about.
  3. The law of inertia is true.
  4. ∴ Change of place of bodies is not a real change in bodies.

And this seems to be correct, if very counterintuitive for small moving objects near the surface of the earth.  But it is really no more counterintuitive that idea of the earth being in multiple kinds of motion relative to the sun and the stars.  And as twisty a history as it is, the word “motion”, which used to mean “change”, came to mean “local motion,” which then turned out not be a case of real change at all!

Certainly the idea that something which changes place is only a relative change is somewhat startling, running counter to our ordinary intuitions, but relativity theory is nothing of not challenging to at least some of our ordinary intuitions.  Neither Aristotle nor Thomas would dispute Einstein.  If change of place is only a relative change and not a real one, it is.  Of course it would be foolish to think Aristotle or Aquinas were fools to think what almost everyone thought until Einstein, namely, that change of place is a real change (although some philosophers did question it; Leibniz’ dispute with Newton over Newton’s concepts of absolute space and absolute time comes to mind—as usual, Leibniz was right, or at least, not wrong).

In any case, the law of inertia doesn’t refute the argument from motion. It simply turns out that the law of inertia shows that change of place or local motion isn’t a case of real motion as Thomas and Aristotle use the word “motion”; or, as we would say today, change of place or local motion isn’t a case of real change.


4 comments on “Does the Law of Inertia Disprove the Argument from Motion?

  1. Thanks for the thought provoking post. Last year I discovered the Aristotelian-Thomistic philosophical system for the first time and it completely changed my life. I’ve been trying to study it as much as I can since then, especially the Five Ways. I have several questions that I wonder if you could help me with?
    First, specifically related to this post, is it possible that even local motion could still be used in support of the argument? For if some object is at rest, inertia states that in order for it to “move” requires some outside force, which, in Aristotelian terms, would actualize its potency for motion. And aren’t the potencies which an object have determined by its form, so couldn’t we say that objects by their very form/nature have the potency to either stay in rest or motion until acted upon by an outside force? So an object in rest will stay in rest until its potency for motion is actualized, but this potency is to *stay* in motion until acted upon again?
    This second question has almost nothing to do with this post but with the Thomistic conception of God in general. I’m convinced, by the Five Ways and St. Thomas’ philosophy in general, that God is Purus Actus, with no potencies. But I’m a little confused about how this relates to the freedom of God’s will. I, as a human, have the potency to do something such as paint a picture. If I didn’t have the potency for this, I’d never be able to do it. I don’t have the potency to jump to the moon, so I’ll never be able to do that. So in order for God to create, would he not have to have, as part of his essential nature, the potency to create, and since God is Pure Act, all of his potencies are necessarily actualized, so my question is, does this not mean that God necessarily had to create, by the nature of his own essence, and does that mean God did not freely choose to create?
    My apologies of these questions don’t make much sense, I’m a bit new to all of this. Love your blog!


    • Eve Keneinan says:

      If local motion really is a Cambridge property is isn’t a case of actualizing a potentiality. A change in local motion, however, is a real change, e.g. a change in velocity or direction. I think modern physics denies a real distinction between “rest” and “motion” because whether a given object is at rest or in motion is relative to some frame of reference. My computer is at rest on my desk. But it is also in motion because it is on the earth, hurtling through space. It cannot be said to be “at rest” or “in motion” simply, without a “relative to” qualifier.

      Your question about God’s freedom is a very good one, and a very old one. It hinges on the proper understanding of freedom, which is most definitely not modern understanding. We think we are most free when we have maximal options. But consider: the wiser we become, the more narrow our options become. Consider a game of chess as an example. There are many moves available, but the chess master sees that only 1 move or perhaps 2 are the BEST moves. The more we become aware of what is GOOD, in a sense, the fewer “options” we have. Does that make us less free? According to the moderns, maybe, but not according to the ancients and medievals. God’s freedom would thus be the ultimate example of this. In a certain sense, God does not select from a variety of options, which to many moderns, makes Him seem unfree. But since God is both all-knowing and perfectly good, it is His nature to always freely choose that which is best.

      We would really have to get into the key concept of the divine simplicity. What would really be the accurate way to put it is, in God, there is no distinction between his acting from the necessity of His nature and His acting freely from His (perfect) will and his acting reasonably from His infinite knowledge and wisdom. What Thomas would say, I believe, is that the distinction between “acting freely” and “acting from necessity” breaks down in God, because in God, these two are one and the same, just as God’s essence, or what He is, is not distinct from his existence, or that He is, as in the case of all other entities.

      So the short answer is no, God’s acting from His nature does not cancel His acting freely, even though in creatures (everything but God), necessity and freedom probably are incompatible.

      One more thing that must be said is that our comprehension of God’s nature will always be imperfect, because we are finite and God exceeds our powers to “comprehend” or capture in thought. Descartes uses the analogy of a mountain: our thought can touch God, as our hand can touch a mountain, but we cannot grasp God in thought no more than our hand can grasp a mountain. So orthodox Christian teaching has always held (1) God is infinitely free, and creates the world freely, and offers His grace freely, and (2) God’s freedom is not arbitrariness; it is the freedom of perfection, so God is by His nature, incapable of e.g. doing evil. But this is not a limitation on God’s freedom; an “incapacity” to do something that is defective is really not an incapacity. I sometimes illustrate this to my students by saying “I can do lots of things that God can’t! I can do evil, make mistakes, fuck up, fail, be ignorant, go wrong, get lost, etc.” Of course, on closer examination, all these “powers” I have that God doesn’t are really not abilities but disabilities or liabilities—in a word, imperfections. It is a trick of language only that makes them seem like powers or abilities.


      • Ok, thank you very much, that really clears some things up for me! So, God in his intellect knows what is best, and his will only chooses to do what is best, so in his nature he only has the potency to create this universe, which is actualized? Also, sorry to bombard with questions, but I have one final one relating specifically to the trinity. If God is one essence with three “persons” where personhood is understood as possessing an intellect and will, why is it specifically three persons, rather than, say, five or seven or any other number? If God has the potency to be three persons in one essence, does he not have the potency to be some other number of persons?


  2. Eve Keneinan says:

    I think the Holy Trinity is at bottom a mystery revealed in revelation which we can accept but not (fully) understand. I would guess that three is the number because it is the simplest number in which love can fully actualize itself. As Aristotle already argued (for very different reasons than theological ones) the number two or a dyad seems to somehow fall short of a true multiplicity: we do not, he observes, say “all two” but “both.” Just as a monad either simply is or circles back on itself, a dyad can result in an endless back and forth oscillation—but if love is, as Christianity teaches, PRODUCTIVE, love wants to BREAK OUT OF any sort of confinement, which would include a dyadic one. Dyadic love can remain selfish love, whereas the image of God is most perfectly found in the family where the love of man and woman generates more than itself in a child, which perfects the love to an even higher degree. Thus the Trinity is not subject to “dyadic confinement” and is itself apt to “break out” beyond itself, and hence God creates the world.

    Insofar as every human person has the potential to become united to God in Christ, there is a sense in which we, as “participants in the divine life” (St. Peter) take part in the life of the Holy Trinity—it would be wrong and heterodox to say that we become additional persons of the Trinity, but something analogous to that does take place—because God shares his infinite well of love and life with us.


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