Drake said that Descartes's whole Principia is unscientific and we read it and it seems so far removed from our science that you can legitimately ask, "Why do we even take this to be science at all.?". I put up the Schuster article giving his Kunian account of why we should take it to be science, but I'm going to give you mine instead of that.

The first thing to notice, and I already said it earlier, is the extent to which this is informed through by observation. Almost nothing in parts three and four are coming out of reason alone. They're all taking constrained laws of motion, which are primarily out of reason. The sling being an exception, and local transfer of motion being an exception.

But, they're taking those as constraints and now asking how can the observed world work given those constraints? And we're simply tacking on features of causation and then figuring out how many other things we get for free, not quite for free but at least follow naturally from what we're positing.

It's the extent to which of the empirical throughout informed empirically that causes me to think that saying this is philosophy is really missing the boat entirely. He's very much looking at the science as much as Kepler, I'll just stay with that for the moment. And now I go back to what I said before.

There's a challenge posed by the failure of Ptolemy. 14 centuries of believing in Ptomely, actually 14 centuries of believing in Aristotle with Ptolemy even though they're inconsistent with one another. The challenge is how can we take observation and turn it into evidence without being systematically mislead. Ptolemy was systematically mislead in concluding that Venus and Mercury go around the Earth.

Okay, it was not crazy, what he said, but he was systematically misled and it was clear to people at the time he was. How are you supposed to prevent that from happening again? And I see Descartes as a response to that, that if you do piecemeal science locally, without global constraints, without fundamental principals constraining every move you make, you run a terrible risk of repeating Ptolemy all over again, of developing a wonderful mathematical theory that will turn out to be, at best, having a loose relation to the world.

Ptolemy did have a relation to the world, but it wasn't the one one thought or worse being systematically misleading about what's actually going on in the world. So I see this as an answer to how to turn empirical information into evidence. My phrase turning data into evidence in a way that you're not gonna be misled.

Protect yourself by demanding foundations and insisting on explaining everything in sight with as much a single unified explanation as you possibly can. With anything that escapes then being a problem. If you view it that way then the question is what's so bad about it? And years ago when Bernard Cohen looked at these notes in an earlier version, you'll see when you buy the translation of the Principia there's a whole paragraph about the notes from this course, and the influence they had on him.

But he laughed at me and said, you can't figure out what to say about Descartes, right? He knew I didn't wanna dismiss Descartes, and he was probably more inclined than I, but I struggled to do it and my conclusion now is that the principle fault of Descartes is he constructs this story, and it just sits there.

He doesn't push it to learn more about the world. If he had started pushing it to learn more about the world, lots of bad things would've happen. His account of vortices would become more and more problematic if he had started doing experiments on vortices around the Earth. If he had tried to find some way to test in the planetary realm.

Say by looking at comets and seeing whether a comet did come within our planetary system. Remember, Tycho had said one did. And that was important for Descartes. If one came within our planetary system there's very little chance it should stay. It should be captured, it wasn't captured, so that's a possibility.

It's not pushing the thing further but resting with it as if this were a semi-finished account. That seems to me the feature, and what I want to say about this, the feature that most distinguishes it from modern science after Newton, of stopping at the first point where you've got reasonably satisfactory results across a broad spance.

Descartes of course was careful to sit metaphysical foundations to all his work in principles of philosophy, which he does openly refer to as physics. There were followers of his who rejected that, in particular Regis and Rohall. Their attitude was much more one in the spirit of the time of feeling uncomfortable appealing to God in order to establish principles of physics.

So the sort of thing they were ready to start from, a purely empirical foundation for Cartesian physics is just this one statement, all departures of anybody from rest or from uniform motion and straight line require an external action or force arising from some other body in contact with it that effects a change in the motion of the body in question by transferring some of its motion to it with the aggregate of their total motion remaining the same after as it was before.

Now take that as the soul foundation for everything in this science. Drop why this is true. Just say, this is the case, we have some evidence for it in terms of transfer of motion, in terms of curve or linear motion, etc. And you've got what actually became, in many circles, Cartesian science.

Cartesian science without the metaphysics of Descartes. But if you think of it that way it doesn't look so terribly different from Newton. Because that's a phrasing of course it's my phrasing. But I made the phrasing as neutral as I possibly could between the two and that's why I put in contact with it in parentheses because that's the only thing that really forces it into being in the Cartesian spirit.