Now, these two laws together, the first and second law, appear to be stating what we now call the principle of inertia. That principle first got called inertia by Newton. By the way, as a slap toward Descartes, you'll see why later. But the question now becomes, should Descartes get credit for the principle of inertia?

Now we saw last week, the passage in Galileo that leads people, scientists, to say Galileo was the person who discovered the principal of Inertia because he says if a flat horizontal with no impediment its' body will move forever. The first person to give an unrestricted form of the principle of Inertia in print was Gassendi in a beautiful monograph called on impressed motion and translation of movables to letters and their letters.

This is his statement. All that has no other aim than to make us understand that most, all that, I'm sorry. Has no other aim than to make us understand that motion impress on a body thru void space, has no problem with that, where nothing either attracts or resist will be uniform and perpetual and that they're from.

We conclude that all motion that is impress on a body isn't itself of that time. So then in whatever direction you throw a stone, if you suppose that at the moment in which it leaves the hand, by divine power, everything else beside this stone is reduced to nothing, it is resolved that the stone will continue it's motion perpetually and in the same direction in which the hand has directed it.

And if it does not do so, in fact it seems that the cause is the admixture of the perpendicular motion which intervenes because of the attraction of the earth. Which makes it deviated from its path etc. Okay? Now what he added that Galileo didn't have? This goes in all directions.

It's got nothing to do with horizontal. So he has real claim to being the first to state the principle. The difficulty is, in a different passage, quoted, I'll have to explain this. Walter Charleton published this huge book, Physiologia Epicuro-Gassendo-Charletoniana, that's supposed to be him but it's almost word for word English translation of Gassendi.

The hardest thing to find is anything Charlotan has added. Charleton. I shouldn't call him Charleton. Charleton.

But here's the quote. Why may you not lawfully conjecture that if the terrestrial globe were of a superficial, exquisitely polite or smooth as the finest Venice glass. Course this is in English.

And another small globe is polite replaced in any part of it's super fist. But generally impaled any way, it would be moved with constant uniformity quite around the Earth according to its first direction. And having rolled once around the Earth, it would, without intermission, again begin and keep coming.

Now that's Cassini saying that. He's in effect saying circular motion is self sustaining. Now he has stated the principle it is not clear to him that there's got to be an external action to maintain any form of curvilinear motion. Descartes alone is the one who emphasized that. And that inclines me to give him credit for it.

The Gassendi two letters by the way, what Gassendi did was to take two new sciences published just before, go out on a ship, and start doing experiments to confirm what Galileo says, like go to the top of the mast and drop an object to see where it lands.

Go into the hall and see whether something moves the same way whether the boat is moving or not moving. It just, all these experiments, all of them concluding, and this is actually a quote from Dugas. How about sure enough you go inside the hull. You drop an object from the mast.

All these things Galileo said are correct. So Gasinde was very much here supporting Galileo and his statement of what we now call inertia is a generalization of that. So what do I mean by the principle of inertia for the rest in this course? The usual way of saying it, the direct statement, anybody if moving at all will continue to move at a uniform speed in a straight line unless it is made to deviate from that motion by an external cause.

Now the form in which Newton primarily uses it and Descartes primarily uses it, it the contra-positive. We wanna draw an inference from the fact that the motion isn't uniform in a straight line. So the contrapositive if a body in motion deviates in any way, either from rest or from moving at a uniform speed in a straight line, then something external to it has caused this deviation.

In other words it licenses a conclusion of an external cause anywhere we've got curvilinear motion and anywhere we've got speeding up and slowing down in straight line motion. That's a very strong license. There's gotta be an external cause now we gotta figure out what it is and characterize it.

Now, I called this a conceptual change, and what I mean by that, in fact what I always mean when I go along with Tom that conceptual changes have been so important in the history of science, is The set of questions that demand answers versus the set of questions, why questions, to which the answer is why not, those change.

So here the question, why does it continue moving in a straight line when you throw it? Answer, why not? Nothing is causing it not to do so. By contrast, why is it moving curve linear? You can't say why not, you've got to identify a cause. That's what I mean by a conceptual change.

We're reconceiving which why questions demand answers and which ones do not demand answers. And I like that generally. I mean, the way I think of concepts is their distinctions. They're not isolated, they're drawing distinctions of various sort. Here we're drawing a distinction between when a question requires a substantive answer, and when it can be dismissed as not requiring a substantive answer.

And that's changed at this point. Okay, even in Galileo we had the impetus being collected, that stays with the body moving. So that's the point I make here. It's the very question Kepler had posed that you're now gonna reject. Why does that body continue to move? Kepler's answer, a flux from the sun.

We're gonna replace it with, why does that body not continue to move at the uniform speed in a straight line? And that's gotta be because of the action on it, something external to it. And that of course means uniform circular motion is not self sustaining. That's what I'm gonna center on, is the principle of inertia.

And my view is one reason it was almost universally accepted as soon as Descartes put it forward Is the climate was totally ripe to accept it. That said I want to give Descartes the primary credit for it because he was the only one who recognized the point about Curva.

Only one to state and emphasize and you saw the emphasis. He says this is gonna so important the continuation and everybody should note it here. He's the only one to do that. And after him, everybody does that.