Philosophy 167: Class 12 - Part 13 - De Gravitatione et Aequipondio Fluidorum: a Reductio of Descartes' Theory of Motion.

Smith, George E. (George Edwin), 1938-
2014-11-25

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Synopsis: Review's Newton's De Gravitatione et Aequipondio Fluidorum; discuses Newton's definition of place, and then uses it to disprove Descartes.

Subjects
Astronomy--Philosophy.
Astronomy--History.
Philosophy and science.
Gravitation.
Physics.
Newton, Isaac, 1642-1727. De gravitatione et aequipondio fluidorum.
Descartes, Ren, 1596-1650.
Genre
Curricula.
Streaming video.
Permanent URL
http://hdl.handle.net/10427/012737
Original publication
ID: tufts:gc.phil167.144
To Cite: DCA Citation Guide
Usage: Detailed Rights
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There's a manuscript we don't know how to date. It's called On gravity and the of Fluids. What's the word I want in English?
Equilibrium?
Yeah, it's really a hyperstatic equilibrium condition. I show you this manuscript to drive a point home. It's in Newton's hand. Very few things we have are in Newton's hand.
When we have them they're almost always marked up because he's making revisions all the time. There are no revisions to speak of here until he gets down here and throws three additional definitions in and puts the word definitions there. This manuscript runs some 30 pages, these the second and third pages.
There are just almost no changes to this. A word here, a word there. We go back 25 or 30 pages until he starts looking like he's drafting it. So, this is something Newton is copying from something else, making a few changes as he does. We don't know when he did it.
He may have done it as early as 1670. Betty Joe Dobbs, the person who first investigated Newton's work in alchemy extensively, argued he did it at the time of the Principia. For a long while, I thought it was an amalgam of early work that he was copying and work around the Principia, because the word inertia appears twice in it.
I've now concluded, from research I've been doing in the last year on the1685 manuscripts that that can't be true. This had to appear before Newton started work on the Principia. I don't know how to date it now. I'm gonna try to go fast. This is the single document that philosophers writing about Newton put their most greatest time into, cuz it's the one place he is really openly philosophical.
And I won't put much weight on it, because I don't know how to date it. I rather suspect it was a youthful work that he thought he could resurrect to make a point about Descartes. You'll see the point in just a moment. But when you go looking at the philosophic literature on Newton, this is treated as the gold mine for how he thought as a philosopher.
And I am one of the few people, Rob Ailef is another, who do not think this way. So one of the four definitions that he puts in there? Place is a part of space that a thing fills evenly, body is that which fills the place, rest is the continuance in the same place, motion is the change of place.
He now uses that to disprove Descartes imaginings, figmenta. And the argument goes as follows. Given Descartes' definitions of place, body, rest, and motion, they're incompatible with his claims about vortices and uniform motion in a straight line. And I'll just quote the portions of this that are most important.
It then follows that any moving thing has no determinate velocity and no definite line in which it is moved. And much more that the velocity of a body without impediment to its motion cannot be said to be uniform, nor the lines straight in which the motion takes place.
Nay, rather that there can be no motion since there is none without a certain velocity and determination. Indeed, it follows that Descartes' motion is not motion, seeing that it has no velocity, no determination, and that by it, no space and no distance is covered. It is necessary, therefore, that the assignment of places and thus likewise, local motion, should be referred to a certain immobile entity, such as extension or space alone in so far as it is regarded as something truly distinct from bodies.
This is the origin of talk about absolute space. And his point is, Descartes can't make sense of the principal of inertia if motion is entirely relative. Because what appears to be a straight high motion relative to one body may not be straight at all relative to another body, depending on how they move with respect to one another.
So it's a reductio of Descartes' account. It's a very good reductio. I'm just quoting the conclusions from it. You can read the whole thing. And that's followed by 13 definitions. I'm not gonna go through them. Force, conatus, impetus, inertia, gravitas. The intention of any of the above mentioned powers, the extension of the above, and finally, the absolute quantity as the product of the intention and the extension in the one I want to single out.
The intention of gravity is proportional to the specific gravity of the body, its extension to the size of a heavy body. And speaking absolutely, its quantity of the body is the product of its specific gravity and bulk. That's the first approximation to the notion of mass that we have.
Now, I can't continue, I gotta let you go home and have a nice Thanksgiving. There's just an enormous amount written about these. I put the whole of the English translation and the whole of the Latin under supplementary material. Next semester you can read Howard Stein on this. I actually think Howard Stein uses it correctly.
But the world is filled with philosophers who have taken DeGraff as telling you what Newton really means by absolute space, absolute time, et cetera. The reason I discounted, and notice Arabell gave you only excerpts from him, reason I discount it, is it could be 15 years before he works on the Principia that he did this, and that gap is huge.
What he's writing here is a diatribe against Descartes' figmenta. It's a powerful diatribe, but that's the most I'm willing to say.