Mitch on Newton

Newton’s Scholium on time, space, place and motion

These are a few general questions that came up when I was reading Newton and Rynasiewicz and Newton’s arguments for absolute motion, as I think they could be reconstructed:

Newton’s aim is to show that true motion must be absolute by showing that it cannot be relative to other bodies. Rynasiewicz, Newton and the lecture slides describe it as to show that motion cannot be determined, or analysed, by only looking at objects and how they move relative to each other. How is that different from the claim that true motion is absolute motion? Is this epistemic aspect weaker, or does this aspect only follow from Newton’s premises?

There are some formulations in both texts which to me seem to indicate that Newton argues for the identity of true and absolute motion. For example, Rynasiewicz states that “the difficulty of (…) so doing constitutes for Newton a strong argument for the existence of absolute space”. The step from absolute space to absolute motion is not far, Newton even defines absolute motion as a change of place in absolute space, early in the Scholium (IV). Another one is that Newton, according to Rynasiewicz, wants to justify his distinction of true and relative motion” – meaning true motion is not relative, thus absolute.

It seems strange to me that Newton defines true motion as the change of place through absolute space. That suggests that he defines true motion to be absolute motion and then uses this definition for circular arguments. That true motion exists is agreed on by all, but absolute space and motion should be his conclusions, rather than dogmatically fixed in the beginning. Throughout the text, both Newton’s formulations and the structure of his arguments look as if this was the case. Some of his premises appear to just formulate negations of Descartes’ views.

Furthermore, according to Rynasiewicz, Newton treats “absolute motion” and “true motion” as synonyms (in 5.3). This does not make sense to me at all. Either that is intended, but then the arguments are circular, or Newton is not clear enough.

I have tried to reconstruct the four arguments from properties, cause and effect, and will also shortly comment on their appeal to me.

Properties 1:

I found this argument the hardest to understand. My first intuitive reconstruction was this:

1.       Property: If two things are at true rest, then they are also rest with respect to each other

2.       If true rest is taken to be relative (in the way Descartes does), then two things can be at true rest, but not with respect to each other (they are then only at rest relative to their direct neighbours but not to each other).

3.       So, true rest cannot be taken to be relative.

4.       If true rest cannot be relative, it must be absolute.

5.       So, true rest must be absolute.

The first premise is convincing, if it is interpreted as the following answer to why one should believe it to be true: It is our strongest intuition corresponding to the way we speak about motion and rest, that if two things are at true rest, they are also rest with respect to each other. This is just what we mean when we say “rest” and “motion”. To state the premise just as a fact is problematic, because it presupposes absolute rest. To defend Descartes, one could then simply reject the premise on grounds of Descartes’ notion of true rest.

But this reconstruction does not seem to really reflect Newton’s intention.

1.       Property: if two things are at true rest, then they are also rest with respect to each other.

2.       This property implies that in order to find that something is truly at rest one has to find that it is at rest relative to something at true rest, which cannot be done by analysing anything around us.

3.       Thus, one cannot analyse something around us in order to find true motion.

Unfortunately, this is the best I could do. This cannot be right, the second premise is clearly wrong: the property does not imply that, because one cannot find something by finding its necessary condition.

Properties 2:

1.       Property: If a part of an object moves, then another part of the same object moves with it.

2.       Two bodies where one surrounds the other one are like two parts of one body.

3.       So, if the surrounding body moves, the surrounded body moves with it.

4.       If true motion is motion relative to its contiguous bodies, then the surrounded body does not move.

5.       So, the surrounded body moves and does not move, which is impossible.

6.       Thus, true motion cannot be motion relative to its contiguous bodies.

It is natural to agree on the property, since a body as a whole either moves or does not move. I think the crucial premise is premise 1, the question whether that analogy really holds.

At first I reconstructed this argument in a different way, which does not seem to be what Newton aims at, but still looks like an interesting challenge of Descartes’ view similar to the above:

1.       If true motion is motion relative to its contiguous bodies, then a body, contiguous with two objects, one of which is moving and the other of which is not moving, moves and does not move at the same time.

2.       A thing cannot move and not move at the same time.

3.       So, true motion is not motion relative to its contiguous objects.

Is that convincing? Could Descartes still claim the predicate “to move” to be a one place predicate?

Argument from causes:

1.       Force impressed on a body is necessary and sufficient for its true motion (or the alteration of it).

2.       If true motion is motion relative to its contiguous bodies, then force impressed on a body is neither necessary nor sufficient for its true motion (the alteration).

3.       Thus, true motion cannot be motion relative to its contiguous bodies.

The question here is whether 1 is true. This is again a premise that Newton just seems to take for granted, but that seems intuitively right, just like the properties in argument 1 and 2.

Argument from effect:

The argument consists of a counterexample on Descartes’ universal claim that all true motion is relative to contiguous bodies.

1.       The centrifugal force is an effect and therefore a necessary and sufficient condition of rotation, which is true motion.

2.       The rotating bucket experiment shows that if true motion is motion relative to its contiguous bodies, then there can be a truly moving (rotating) object, that does not show any centrifugal force (water at the beginning) and that there can be a truly resting object with clear signs of centrifugal force impressed on it. The effect is neither necessary nor sufficient here for motion.

3.       So, true motion cannot be motion relative to its contiguous bodies.