Where inertia comes from (2)

Pre-Newton (for us “pre-science”)

1.) Physics is the science of motion

2.) Science is an ordered presentation of cause-effect relations.

3.) So physics is an ordered presentation of movers in their relation to moved things. We relate all motions (even the births of things) to the rotation of the stars, the impression of forces, or the action of energy on initial conditions.


1.) Physics is the science of motion.

2.) Motion is (or reduces to) action in space.

3.) Space is, of itself, without fixed borders, reference points, or landmarks.

4.) So of itself, motion is an action with no fixed borders or reference points. These must be provided by pure stipulation.

5.) Therefore, whether something is at rest or at motion is a matter of pure stipulation. Of themselves, motion and rest are indistinguishable. Call them states. 



  1. November 11, 2015 at 8:27 am

    You are neglecting to mention here that these changes in description did not happen by chance, but at least in part because of how things were observed to actually behave.

    • November 11, 2015 at 12:02 pm

      It’s odd to describe this as “neglecting to mention”. If you stop describing a group of men as plumbers and start describing them as citizens then, if both are true, this will result from “observations of how how the things were observed to actually behave.” But outside of a contrived and obscure circumstance, talking about plumbers is not an instance of neglecting to talk about citizenship.

      • November 12, 2015 at 7:54 am

        If you don’t intend to talk about the objective facts that resulted in the changes in description, then calling your discussion “where inertia comes from” and not mentioning them is indeed neglecting to mention something, since it does not in fact explain where inertia comes from. Of course since this was a second post with this title it is possible that you were simply leaving that for later.

      • November 12, 2015 at 9:51 pm

        If you don’t intend to talk about the objective facts that resulted in the changes in description

        You are assuming that it was the objective facts that resulted in the changes in the description rather than the changes in the description, by changing things like what people looked for and how they measured things, resulted in the discovery of the facts. There is in fact a very good argument in history of science that the latter is the case.

      • theofloinn said,

        November 13, 2015 at 9:01 am

        But no one to this day knows what inertia is, other than the Latin for “laziness.” Mach’s Conjecture was that inertia was the sum gravitational effect of the rest of the universe; but how do you test something like that? Everyone back in the day knew it took effort to get something moving; though if there is no distinction between motion and rest, it is hard to see how resistance-to-being-moved works. Buridan said that once an impetus (momentum) is impressed, it would continue indefinitely, unless it encountered a contrary impetus. This also sounds a lot like inertia.

        Pre-Newton physics held that motion tended toward rest. Post-Newton — I mean _very_post-Newton — we say that dynamic systems tend toward an equilibrium state. Oh, _that’s_ different. And Oresme, citing Wiltelo, referred to the relativity of motion in pointing out that whether the heavens revolved or the earth rotated depended on whether you were affixed to the earth or to the sphere of the stars.

        But if kinesis (“motion”) is a _change_ to something a body already possesses and a body already possesses a rectilinear motion, then any _change_ to that motion, as Thomas said, requires an outside force. We prefer to call that change an “acceleration”:but we say the same thing about it today.

  2. November 12, 2015 at 8:04 am

    But talking about objective facts is exactly what #2 does. What is said there is much more solid fact than, say, a thought experiment about a cart on a frictionless road, and it is no less a fact than noticing how a ball falls on an airplane. Further, I take it as a more fundamental fact than either of these.

    I didn’t get what you were looking for the first time around. I saw the post as saying, in effect, “this is physics before inertia, and this is physics afterward. Look at the key re-evaluation of motion from a moved-mover relation to something in space”. I thought you wanted a fact that people pointed to in order to explain why one way of looking at things was better than the other. I gave none. But I did give a fact that shows where the idea arose from, but I took it as simply different and not as competing. Uniform or unchanging spatial motions must be the same as rest.

  3. thenyssan said,

    November 12, 2015 at 8:26 am

    Doesn’t 4), or the move from 3 to 4, provide an ordered presentation of cause-effect relations? It seems there’s a hidden “space gives what it has to action, i.e., nothing.” So the claim is that space informs action, and in its deficiency we stipulate that form.

    I love your pure physics posts.

    • November 12, 2015 at 8:55 am

      That was the idea. I think everyone has a tacit agreement with Aristotle’s claim that motion is always relative to the immobile, and so if we put motion in space we lose all fixed points of immobility. Before Newton, we assumed either that there was a structure to the background in which things moved (there wasn’t, it turns out) or that motion was an asymmetrical relation of mover and moved, and we could put these in order, even in the absence of an order of change of place.

      • thenyssan said,

        November 12, 2015 at 5:19 pm

        You’re going all transcendental Thomist on me. It hurts my head.

        My knee jerk refuge–it’s another proof that the soul is infinite and immaterial.

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