Philosophy 167: Class 8 - Part 7 - Two Hard Cases for Mechanical Philosophy- Descartes' Accounts of Magnetism and Gravity.

Smith, George E. (George Edwin), 1938-


  • Synopsis: Review of Descartes' accounts for magnetism and gravity.

    Opening line: "Obviously, a terrible difficulty facing the mechanical philosophy was magnetism."

    Duration: 7:20 minutes.

    Segment: Class 8, Part 7.
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Obviously, a terrible difficulty facing the mechanical philosophy was magnetism. Gilbear, the model of the most successful Renaissance naturalist has all these properties, fills the whole book of all these properties of magnets, talking about the power of load stones, etc. The word magnetum in Latin is translated just as much load stone as it is magnet, the two were not distinguished at the time.
We didn't have electromagnet's, etc, all we had was iron. So Descartes, whatever fault Descartes had, he was not intellectually dishonest. So he not only takes on in the the really hard problem of showing he can give an account of magnetism, he does something much more than that. He gives an account and then goes through all, one by one the phenomenon, the Gilbert treated, showing his account covers.
So he could not do more. Now, what's the account? In load stone, or in iron, whatever you wanna call it here, there are pores. First is the solid matter, and the pores are threaded. They have left threads and right threads. And there are slivers, and they're threaded likewise.
And when a right threaded sliver tries to go into a left threaded pore, it pushes, cuz it can't go in, and we get repulsions. But when we go the other way, it can go in and we get attraction, because there's now nothing resisting and the two come together.
That's the basic model. And that's what you're seeing here, left and right, this course is drawn from the Principia. Left and right, little magnetic particles too small for us to see, pores too small for us to see, but giving you the north south phenomena. And then he just goes one by one.
I'm not gonna bother to do this. One by one, through the different phenomena, showing he can give at least an equalitative account to what's going on. And to give you a feel, alignments in for how seriously people talk this, alignments in the new essays on human understanding has replied a lot, cites this as a great example of why Descartes was so good at hypothesizing things.
Because of course from a mechanical philosophy point of view, what else can be happening, right? How else are you gonna get north south effects etc.? This won't matter to us much, Newton has a couple remarks about magnetism but he never honors this model even with a comment. Descartes own comment though is fairly striking.
It is less consistent with reason to imagine something unintelligible, action at a distance, in order to appear to explain where a faction by a merely device that it is to conclude from the fact that bodies become rarefied, that they contain pores or interstices which grow larger and that some new body approaches to fill these pores even though we may not perceive this new body through any of our senses.
Okay, it is less consistent with reason to postulate powers than to postulate unseen entities that do the job. Okay. And he's not being light here. The other hard case is gravity. And, I had you read the entire passage on gravity, and on weight. I'm putting on supplementary material, an article by Peter Moore discussing his theory of gravity more fully.
It's complicated. The basic idea is surrounding us well, let me start. There are three kinds of matter. Terrestrial matter, which is even the air, etc., the stuff we come in contact with, can feel etc.. Smaller than that are two kinds of celestial matter. One is globules, which are spherical and that move at very great speeds.
And then there is a third kind of matter which can take every shape you can imagine. Because what happens is it gets formed into these crazy shapes to fill the so that there are no vacuums. Okay, it gets rubbed into these crazy shapes. And now the basic idea is the globules are moving around the Earth at very, very great speed, and you get a buoyancy-like effect from their doing so that creates weight.
Now, that's not quite the right description, so I'm gonna give it to you more fully, and I'm not gonna read this whole passage out, but I wanna try to do my best with this. The picture ends up being that gravity has to be a push on us. Okay?
And the push, therefore, has to take the form of some unseen matter that is trying to descend down and pushes on the solid matter, the terrestrial matter in bodies. It can't just move through the interstices cleaning and instead starts impacting and pushing down. That's the ultimate model, and it's not just his, even Newton flirted with it at one point, Hoygen stayed with it his entire career.
And the background to it is if gravity has to be by contact it has to be something pushing. And since we can't see it we have to postulate it etc.. The important and I'll let you read that on your own. I've marked off on the red. The important conclusion for purposes as this course goes on.
Weight does not correspond to the quantity of matter in each body. Weight corresponds to the quantity of solid matter in each body. So two different things of the same volume have exactly the same quantity of matter in them, but not the same quantity of mass. A gold cube has much more, a higher quantity of solid matter than a balsa cube of the same size.
That's the idea. Now again, he goes on some length. I've only lifted out two of the articles and then thrown it at Peter Deer because I don't wanna spend a lot of time on this now. Other than we're gonna see it coming up again and again, because under the mechanical philosophy, it's very hard for any gravity to be anything else but some version of this.
That's why people kept coming back to it.