The First Way and inertia – the unification response

The First Way sees all motion as something caused and at least as initiated. Inertial motion, so it seems, need not be caused or initiated. There are many lengthy responses to this, but one shorter one that I haven’t seen might be called the unification response, since it turns on the fact that inertia makes a unification between motion and rest:

To consider something so far as it is moving inertially is to consider a motion so far as it is no different from rest.

The First Way does not consider motion so far as it is no different from rest.

As long as one admits that there is at least one coherent way of considering motion as different from rest, then the First Way can proceed just fine.

Newton would not agree that inertial motion is exactly the same as rest: he argued for an absolute space that gave an ontological reality to inertial motion, even if one could not mark it out with a metric. This absence of a metric is itself very problematic, but with the loss of absolute space, inertia comes to this.

Another difference is that Newton saw inertia as only active in the change of motion, not in the continuance of it. But this latter way is exactly how we have to see it in order for it to be an objection to the First Way.





  1. theofloinn said,

    June 3, 2014 at 4:56 pm

    Mach’s conjecture is that inertia is simply the net effect of the gravitational attractions of the rest of the universe. In this sense, inertial motion is still caused by the other bodies in the universe.

  2. Joshua said,

    June 6, 2014 at 12:46 am

    While rather simplistic, it seems to me that on the face of it there are two possibilities…either uniform rectilinear motion is continually caused, as Aquinas believed. And in some way this would necessarily follow if in moving the object gain any new actuality, the devil would be in the details.

    If, however, this sort of motion that Newton was concerned about and Einstein unified does not involve gaining any new actuality, but is a state in which the object remains unless acted upon, we still merely have said that this type of locomotion is not motion in the sense Aquinas is using, but, e.g., to be accelerated would be a motion in that sense, since its state would thereby be changed, at least with regard to the same plane of reference. Hence we still observe motion in the sense the first way invokes.

    Now I favor the first position, and I would love to finally figure out the devil in the details of it. But as far as inertial motion being an objection, it just seems quite clear to me because of the 2nd possibility (or indeed the existence of alteration or growth) that is falls flat, no?

    • June 6, 2014 at 7:05 am

      Right, that’s exactly what I wanted to argue. If it’s not caused, it’s not motion considered under the ratio that the first way considers it.

  3. Timotheos said,

    June 11, 2014 at 11:02 am

    I’m inclined to think that there is a subtle equivocation going on between Aquinas and Newton.

    When Newton says that inertial motion can continue forever without a cause, he says this applies only in the case in which that motion is in a vacuum.

    However both Aquinas and Newton are explicit in the fact that they think a vacuum is an impossibility. (And I believe that Aquinas thought is was self-evidentially so, if I’m not mistaken)

    Thus, we can take Aquinas to be claiming that an eternal continuous motion without a continuous mover is impossible because, otherwise, the motion would stop at some point due to friction and thereby not be eternal.

    Whether or not Aquinas would accept that motion would continue eternally in a purely hypothetical vacuum is besides the point (although it does seem that it would line up with his notion of an impetus); the main point is that these claims are not talking about the same thing and are thus not contradictories.

    • June 11, 2014 at 12:08 pm

      But inertia is not impetus. Impetus is a positive form that accounts for why an object is in motion. Inertia is a resistance to change. Inertia activates only at the moment in which some force endeavors to change the state of the mobile – i.e. to cause change in the second time dimension.

      This is one of the things that makes Newton’s position so counterintuitive – he simply sees uniform motion as having no cause at all, either of its maintenance or even its acquisition. There is no more need to posit a cause of such a state for him than to posit a cause for absolute rest. This is the radical implication that we have to either take or leave – take it and we need to allow for some sort of identity of motion and rest; leave it and we have to utterly throw out the idea of inertial motion. The impetus theory, seen from this angle, is a denial of inertia by being a refusal to accept that uniform motion needs no cause.

      I’m seeing if one can make sense of this seeming uncaused state as the self of the inanimate, which makes inertia a struggle to preserve oneself.

      What this means, contra Aristotle, is that the inanimate as such has neither motion nor rest as natural to it as an operation. What is natural is the interactive, that is, to locate oneself in a system and to take ones identity from it. This system determines whether motion or rest will, in turn, count as natural to the body.

      I’ve got a ways to go with this.

      • Timotheos said,

        June 11, 2014 at 2:26 pm

        “But inertia is not impetus.”

        Sure, inertia is not impetus and I never said it was. But can we not say that inertia “protects” the impetus?

        Here’s how I’m seeing it: inertia is a description of the tendency of bodies to avoid change and separation. When an external force is applied to a body, thereby introducing an impetus, the body resists the introduction of the impetus due to inertia. Once the power of the force overcomes that of inertia, the inertia will preserve the body from losing the impetus until it is faced with some other power, like the inertia of a different body (i.e. resistance). The impetus will act without changing, protected by the inertia, unless the body’s inertia is overcome, in which case the power of the impetus will be weakened (presumably by the resistance from another body).

        So it seems that the impetus needs no more cause, in a Newtonian sense, than does inertia; we may regard them as either natural or quasi-natural, and thus, formally caused.

        Putting it shortly, it appears that there is some more equivocation here, this time between the word caused in their respective Thomist and Newtonian senses.

        But then again, I could be missing something and am open to being wrong…

      • June 11, 2014 at 3:49 pm

        I’m making a stronger point than that inertia isn’t impetus, or that the doctrines are merely different: I’m claiming that impetus is a causal explanation for something that is, in the doctrine of inertia, without a cause. So far as Newton is concerned, impetus in this domain is utterly superfluous.

        (For whatever reason, I’m feeling unusually emphatic today. By nature I’m much less like this. I suppose we have to both suffer through it.)

        “Impetus” presumably is a synonym for “vis” or “force” here, that is, it is a sort of cause. But, as far as Newton is concerned, no force is necessary to keep an object in uniform motion, since it is a state no different than if the object were floating at rest in empty space. If we say this latter is “not acted on” then so neither is the other.

        This is a serious point of contention, but it seems to be at the basis of all the physics that came after it. I don’t know if we’ve yet come to terms with it, to be honest. All modern physics rests on the idea that motion and rest are both the same, both uncaused.

  4. Timotheos said,

    June 11, 2014 at 11:40 pm

    “(For whatever reason, I’m feeling unusually emphatic today. By nature I’m much less like this. I suppose we have to both suffer through it.)”

    No problems on my end as long as you’re willing to suffer with me and my replies.

    “I’m claiming that impetus is a causal explanation for something that is, in the doctrine of inertia, without a cause. So far as Newton is concerned, impetus in this domain is utterly superfluous.”

    Ok, that’s fine; I agree that Newton both sees inertial motion as a changeless state and that it needs no cause. But I think an important issue turns on the fact that Newton is operating from a deficient notion of cause; he sees causation, like most early moderns, as purely efficient and disregards or at least ignores the other types of causes. Thus, when Newton says inertial motion has no cause, he means it has no efficient cause, and he doesn’t appear to have the conceptual apparatus to respect any other kind.

    So, both Aquinas and Newton, at least on one interpretation of St. Thomas, can agree that such motion needs no efficient cause.

    However, this does not remove the need for either formal or final causes, and it appears to me that inertial motion is going to need to make reference to formal causation to be coherent.

    Indeed, treating inertial motion as formally caused by an impetus would preserve both of Newton’s claims that inertial motion is a changeless state and that it has no efficient cause. Now this might not be an answer that Newton would be comfortable with, but I see no reason why this should be seen as an unacceptable solution, especially if you have already think the self-evident principle of causality is true.

    And really, how is one to make sense of inertia without formal causation? It seems to me that if there is no need for an impetus to cause inertial motion, neither is there one for inertia to cause resistance.

    Again, I’m not particularly attached to this interpretation of inertia; I just don’t see any reason why the solution should need to be any more complicated than this.

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