Spinoza tries to do his best with these rules. I'm gonna put all of part two of Spinoza's Introduction to Descartes' Philosophy on supplementary material. I wanted to do it today but the copier broke so I have to do it separately but I'll put it up tomorrow. But he ends up supporting De Carter at least giving a rationale and proposition 23 states that rationale.

When the modes of a body are forced to undergo a change, that change will always be as small as possible. And notice his ending of the whole of part two on motion. Since these remarks relate to bodies which are called fluids, it follows that fluid bodies are those which are divided into many tiny particles moved with an equal force in all directions although these partials can not be seen by the keenest eye, still what we have now clearly demonstrated should not have to be denied for such subtlety of nature is can not be determined or detained by any thought to say nothing of the sense is sufficiently over come by the remarks and propositions ten and eleven in part two, so you can go back and look at those.

When I put it up, if you're interested. Descartes' own defense of the fact that this doesn't work, or he's already alluded to it and the demonstrations of this are so certain, that even if experience were to appear to show us the opposite, we would nevertheless be obliged to place more trust in our reason, than in our senses, okay?

Indeed, experience often seems to contradict the rules I have just explained. That's in the French, okay? It does, experience is contradictions all over, however, because there can not be any bodies in the world which are thus separated from all others and because we seldom encounter bodies which are perfectly solid it is very difficult to perform the calculation to determine to what extent the movement of each body may be changed by collision with others.

For before we can judge whether these rules are observed or not, we must simultaneously calculate the effects of all those bodies which surround the bodies in question, and which effect their motion. These effects differ greatly depending on whether the surround bodies are solid or fluid. So the problem here is there's all sorts of action going around any sphere.

It will very often be needed only one more globule to hit it to put it into motion, okay? Because so much is already tending to put it into motion. Problem is, we can't see these we can't calculate them we're in trouble but we shouldn't worry about that because reason shows us this has to be true anyways and therefore we're home free.

Okay, different people reacted in different ways to this but the point I tried to stress is we're gonna see a series of solutions to this problem as the course unfolds. And by the way it's the modern textbook solution emerges right away from Hoygens. The problem is that assumes perfect elastic bodies, there are no such thing.

So the modern solution that doesn't assume perfectly elastic bodies is Newton's The Principia.