Principle of Motion versus Inertia

This post will be about a recent paper by Feser, “The Medieval Principle of Motion and The Modern Principle of Inertia“. Feser argues that contrary to first appearances, the principle of inertia in Newtonian physics is not in contradiction to the corresponding “principle of motion” in Aristotelian metaphysics. He defines the two principles as follows:

  • The Principle of Motion: “Whatever is in motion is moved by another”.


    The Principle of Inertia: “Every body continues in its state of rest or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it”.

I note that the conflict between the two lies in how they seem to imply other things affect motion. The Newtonian principle of inertia maintains that a body maintains uniform motion when nothing external acts on it, while the Aristotelian principle of motion that a body maintains uniform (or any) motion because something external acts on it. To succeed, Feser will need to show this is not really what they say. 

Formally Consistent?

Feser notes that the Newtonian Principle of Inertia only denies any “external forces” are acting on the body during the inertial motion. This leaves the “formal” possibility of having some other “mover”, which is not an external force (or an object exerting an external force), that is “moving” the object along the inertial motion.

The problem here is that this other “mover” is simply denied by reasonable formulations of the Newtonian principle. While Feser’s formulation is that an object continues in a straight line unless it is acted on externally, an equally reasonable formulation is that

  • The Principle of Inertia 2: A body that is not acted on externally [a “free particle”] will continue in uniform motion.

The core of the dispute between the principles is whether change requires external influence. This conflict cannot be brushed aside by careful phrasing to avoid “formal” conflict between the statements.

So Feser can combine the two, but not in a satisfying manner. He can combine them only by invoking “non-physical” causes which do not change velocity, but rather sustain it – I shall call these Sustaining causes. These causes are not invoked by the Newtonian principle and have no place in Newtonian physics. This “formal” success is achieved only by needlessly multiplying entities.

Inertia as Stasis

Feser’s strongest argument proceeds by two main steps. It begins by explicating the principle of motion in Aristotelian thought. Aristotelian “motion” means change, and change is the transition from potentiality to actuality. So the principle of motion “really” says that

  • Principle of Motion 2“Any potency that is being actualized is being actualized by something else (…that is already actual)”.

Now the second leg of the argument is that inertia in modern Newtonian physics is seen as a “state”; it would be more accurate to say that motion in a certain relative velocity (i.e. at a certain velocity relative to a particular observer) can be seen as a metaphysical “state” of the object [1]. The Principle of Inertia then indeed says that any change to this relative velocity will occur only by the influence of another object (exerting an external force on it), just like the Principle of Motion 2 says. So the two principles are actually compatible.

The problem with this interpretation is that it does not account for the change in the particle’s position. It shows that the changes to the particle’s (relative) velocity correspond to the principle of motion, but not that the changes to the particle’s (relative) location do.

Can Feser successfully argue that change of location during an inertial motion is not an actual change in the Aristotelian sense? I doubt it.

Change of velocity and change of location are of course related. To claim that change in location isn’t really change Feser would have to argue that location isn’t a real property; it is velocity and the passage of time that determine it. This is opposed to both Newtonian physics and common sense. 

Further, change of location seems by itself to be genuine change. Physically, it is a “real” property – an invariant property, not depending on perspective (much like relative velocity or whether a motion is accelerated or not). Metaphysically, it appears absurd to the highest degree to claim that a sudden change in location is not change, and I can’t see why a smooth and uniform change will be any different.

So it appears to me that this line of argument fails. 

Inertia as Natural Motion

A second argument that could have been strong is the suggestion to see Newtonian inertia as analogous to Aristotelian “natural motion”. Aristotle believed objects naturally move towards their place – stones move down towards the center of the earth, fire moves up towards the heavens, and so on. Feser concedes this belief is false, but notes that this “natural motion” does not require “something extrinsic”. According to Aristotle and Aquinas, at least. If we replace the natural motion to be inertia instead of towards the proper place, then, it appears inertial motion could proceed without “something extrinsic” as well.

It appears to me, however, that this amounts to saying that “natural motion” or “inertial motion” can be actualized without being actualized by “something else”! Feser unfortunately does not explicitly explain how this notion of “natural motion” fits with Principle of Motion 2. He says only that “a body will of itself tend to move towards its natural place by virtue of its form” [emphasis added] – but the object’s form (it’s essence, or structure) isn’t “something else… that is already actual” [emphasis added], as Principle of Motion 2 requires.

Without such an explicit explanation of how natural motion conforms with Principle of Motion 2, this argument fails.

Inertia as Change

I have argued that inertial change of location cannot be seen as a “state”. The only way left to Feser is to treat it as real change, then. Here Feser’s arguments become quite convoluted, however.

Feser first considers attributing the motion to its initiator, but dismisses this option, seemingly because the mover will no longer be actual. He argues that the motion can nevertheless have a metaphysical cause. Such a cause can be internal or external.

Considering an internal cause – an “impetus” imparted to the object upon its acceleration or generation – he raises two problems: a finite object can have only finite qualities, and such an impetus will apparently be an infinite one; and a finite impetus will change (since apparently finite causes that bring changes undergo change), so we’ll need to explain the impetus’ own change and thus our explanation will not advance us anywhere.  

I note that these objections invoke yet further Aristotelian principles. More importantly, the very idea is in direct contradiction to the principle of motion! The whole question is whether the change requires an external influence.

Feser then reaches the most stupefying part of his argument. Considering external causes to real change, Feser argues that since inertial movement is eternal (in potential) what is required to sustain it are “necessary beings” in the sense that they “have no natural tendency toward corruption the way material things do”. He concludes that

“Hence, the only possible cause of inertial motion – again, at least if it is considered to involve real change – would seem to be a necessarily existing intelligent substance or substances …(Unless it is simply God Himself causing it directly as an Unmoved Mover.)”

I’m going to simply ignore the “intelligent” bit there, as that is not borne out by Feser’s argument above (although it might be by yet further Scholastic principles). I note, however, that Feser is reduced to hypothesizing non-physical sustaining causes to maintain the principle of motion. Which is precisely where we started.


I have shown that Feser has to explain the change in location during inertial motion as real change. He cannot explain it as stemming from an internal cause, as (notwithstanding his own arguments) that would violate the principle of motion. He cannot explain it with an external physical cause, as that implies contradicting the principle of inertia. He is reduced to invoking hypothetical “metaphysical” external causes such as God or necessary substances, whose causal effect is not a force. The only such possible cause is a sustaining cause – positing that something needs to cause the object to maintain its current velocity.

In short, Feser fails to combine the two principles in a satisfying manner. Combining the Aristotelian principle of motion with the Newtonian principle of inertia is only possible if one is ready to assume ad hoc redundant invisible sustaining causes.

Not A Metaphysical Principle

Finally, I would argue that Feser’s position is self-defeating. I have already showed that he must commit to additional external causal entities. But the Newtonian physics is fully consistent without assuming these other entities. Hence, the principle of motion cannot be a metaphysical principle, since it is possible to conceive of change without it – either by invoking internal causes such as impetus, or by declining to demand a cause to explain inertial motion at all. 

Appendix: Some Weak Arguments

There are several other arguments Feser raises, that I think are very weak and don’t fit the above scheme, so I’ll take them on in this section.

Feser argues that while Principle of Motion 2 speaks of actualizing potentials, the Principle of Inertia formally doesn’t, so there isn’t a formal conflict. Well, the conflict is substantial so cannot be wiped away by word games. If the principle of motion is to be put in the language of actuality and potentiality, then the principle of inertia should likewise be put in a similar language or else the principle of motion’s implication in Newtonian terms need to be spelled out for them to be comparable. You can’t demonstrate there is no conflict by putting the principles in different languages!

Feser also argues that the Newtonian principle of inertia is a principle of physics, describing how the world really acts. The Aristotelian principle of motion, in contrast, is a principle of metaphysics which gives an account of the “intrinsic nature of that which moves”. 

I find this argument rather obtuse. Feser appears to attempt to reconcile the two principles by restricting the principle of inertia to talking about the mathematical description of motion, while maintaining that the principle of motion discusses the causal relations that underlay that description. However, the conflict between them is about whether or not something external acts on the object during inertial motion, so the question is about causal relations in the first place. 

Feser also addresses the modern Relativistic idea that the whole world – past, present, and future – exists timelessly together as spacetime. He correctly notes that in Aristotelian terms, this means that the world is entirely actuality, with no real potential and no real change. Feser argues that the principle of motion will be relevant in two ways even in this scenario, but he’s mistaken.

First, he argues that “change really occurs at least within consciousness itself”. But on the contrary, the Parmenidean/Einstenean view is that change doesn’t “really” occur within consciousness – rather, there are different states of consciousness at different places in the man’s worldline.

Secondly, he argues that the laws of nature governing spacetime are contingent, and hence “are merely potential until actualized”. But, in this Parmenidean view potential and the passage of time are an illusion. There is no “until”, nor are the laws “contingent” in the sense that they are actualization of a wider potential. There is simply reality, as it actually exists. There is thus no room for the all the actual to be an actualization of a potential.

The question is, however, besides the point. It does not bear on whether the two principles are compatible.

Similarly, Feser notes that for the Aristotelian what exists are “concrete material substances with certain essences, and talk of “laws of nature” is merely shorthand for the patterns of behavior they tend to exhibit given those essences”. He fails to note that for the Parmenediean, the same is true minus the essences. Talk of “essences” is redundant; the laws of nature suffice to describe the patterns of behavior, and thus essence is made redundant and dismissed as empty metaphysical speculation and dogmatism.

He also argues that for the Thomist things like fundamental particles require an (external) explanation about what keeps them existing. That may be true, but for the Parmenedian this is no need for such an explanation – what exists exists as spacetime, explanations are within this spacetime not about it.

[1] This is not what the physicists would call a “state”. The physical state of an object in modern Newtonian physics consists of both its position and its velocity at a particular time. 


3 thoughts on “Principle of Motion versus Inertia

  1. Formally consistent is ruling out a contradiction. Which is entirely different than ruling it out.
    If I understand the position correctly. He holds that behaving by the law of nature is itself not an explanation. So just invoking Newtons law, is similar to invoking magic. Which both of you don’t want to invoke. The question is, is there something inherent in nature that causes it to act in such a manor? If not why does it behave so. If yes, what is it?

    You argue “This “formal” success is achieved only by needlessly multiplying entities.”
    He says “One might also ask whether such a mover, whatever it might be, really serves any explanatory purpose, and thus whether we ought to bother with it given Ockham’s razor”
    You are cheating by raising a question which Fesser himself raises.

    “Can Feser successfully argue that change of location during an inertial motion is not an actual change in the Aristotelian sense? I doubt it.”

    I think your mistake lies in this statement
    “Physically, it is a “real” property – an invariant property, not depending on perspective”
    I’m pretty sure the physicists would disagree on this one. Fesser even quotes Lee Smolin.
    “Being at rest becomes merely a special case of uniform motion—it is just motion at zero
    How can it be that there is no distinction between motion and rest? The key is to realize that whether a body is moving or not has no absolute meaning. Motion is defined only with respect
    to an observer, who can be moving or not. If you are moving past me at a steady rate, then
    the cup of coffee I perceive to be at rest on my table is moving with respect to you.
    But can’t an observer tell whether he is moving or not? To Aristotle, the answer was obviously
    yes. Galileo and Newton were forced to reply no. If the earth is moving and we do not feel it,
    then it must be that observers moving at a constant speed do not feel any effect of their
    motion. Hence we cannot tell whether we are at rest or not, and motion must be defined
    purely as a relative quantity.”

    In other words. Uniform motion is not change at all. Unlike what Aristotle thought.
    It appears that your statement is not just against Fesser, but against the current understanding of physics.

    Besides Aristotle doesn’t have a problem with change in location has he believed that things have the nature to move towards the center of the earth. So it doesn’t look like change in location is a problem at all. Which leads me to the next point.

    You say “It appears to me, however, that this amounts to saying that “natural motion” or “inertial motion” can be actualized without being actualized by “something else”! Feser unfortunately does not explicitly explain how this notion of “natural motion” fits with Principle of Motion 2.”

    The problem here is that you are asking him to explain Aristotles position. Which you should be able to find in Aristotles writings. So I don’t think you can count your not understanding Aristotles position against Fesser. Since this point wasn’t the point in question.

    • To tthe best of my recollection –

      Feser isn’t arguing against laws of nature.

      If Feser indeed raises Occam in this context,then I should have indeed noted it. Sorry for not doing so. It doesn’t change the fact that Occam indeed applies.

      Change in RELATIVE position is a real change. That is what I meant. Very poor phrasing, I’m afraid.

      Explaining and justifying Aristotle is Feser’s point. He failed to explain how natural motion is compatible with the primciple of motion.

      • If relative change is change what changed A or B?

        Aristotle agrees that motion does not necessitate change of type from potential to actual. As he holds that there is some form of motion that is natural to it. He incorrectly thought it was toward the center of the earth, but Newton corrected that. So according to Newton the natural form of motion for a body is its inertial state. Which when relative to x is V. So long as long as V didn’t change there was no real change.

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