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So Gallileo claims against Aristotle matter doesn't strive left to itself.

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It's in it. It just continues in a uniform state of motion till acted upon by force.

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So the sledge will just keep going in a straight Duret straight line at the same speed unless it's acted upon by force.

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It is, of course, acted upon by two forces, air resistance and friction with the ice.

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So it eventually slows down. Crucially, the heavenly bodies are not composed of a special kind of different stuff.

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It is not true that the moon and Jupiter and so forth have this consist of perfect aether.

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Quite different from anything on the world. No. Galileo wants to say the moon is made a rock just like the earth.

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It's got mountains and valleys and all the rest. It's made of ordinary stuff subject to the same laws.

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But why then does the moon orbit the Earth?

10
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And why did the planets orbit the Sun? OK, we've got a problem here.

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I mean, the sledge seems fine, right? You got. Moves along. Constant direction.

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Constant speed until acted on by force.

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And Galileo is saying Aristotle's got it wrong. The natural motion of a physical thing.

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Something made, if you like, of earth. The natural motion is not towards the centre of the earth.

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The natural motion just keep doing what it's currently doing. It's just inert.

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It carries on doing that until something acts on it to change it.

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Fine. Now explain why the moon orbits around the earth.

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Because here's the here's the moon. According to Galileo, the moon is made of the same kind of stuff.

19
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Why doesn't it just keep going in a straight line? Why doesn't it when it's travelling in this direction?

20
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Just keep on it doesn't it goes round y.

21
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So a big problem remains in Stets.

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Renny Descartes, commonly known as the father of modern philosophy,

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certainly a fundamentally important philosopher of the period and of enduring importance.

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You'll be reading Descartes meditations at some point. You'll be reading some bits of it for this course.

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And I hope you'll read the whole of it because it's a good read. It's one of the best written works of philosophy probably ever written.

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Well, he attacks the Aristotelian tradition. He uses, in effect, the sceptical problem of the criterion.

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Descartes, the very famous for introducing his discussion through scepticism.

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He raises these sceptical problems and says the only science worth worthy of worthy of belief is one that can withstand scepticism.

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Why does he do that? Well, the main reason, I think, is to kick the Aristotelian tradition into touch,

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because his main opponents would be people who would be appealing to Aristotle. So Descartes says, well, it's no good giving me tradition.

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I've got to have something that will withstand the arguments of the sceptics.

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Just appealing to tradition doesn't do that. He builds on Galileo's mechanical philosophy, as we'll see.

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But he makes room for mind. So he goes along with Galileo when he comes to the physical world.

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The physical world is simply consists of bits of matter, if you like, bashing into each other.

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But he says the mind is different. The mind is immaterial. It's not matter.

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And it's quite distinct. So very, very quick summary of his epistemological approach.

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He's looking for a reliable basis for knowledge that can withstand the sceptical arguments.

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He famously comes up with this claim. I think, therefore, I am says at least I can be certain of that.

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I can be certain of my own existence. I can be certain of my own thinking. The sceptics can't destroy that.

40
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Okay. What is it about that that makes me so certain? Ah, I clearly and distinctly perceive that it's true.

41
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Okay, so it looks like we can rely on clear and distinct perception.

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Then he goes on and gives it an argument for the existence of God, which he claims is clearly and distinctly perceived.

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And then he concludes that because God exists, God must be perfect, God cannot deceive us.

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Therefore, he must have made us in such a way that our faculties are reliable if we use them properly.

45
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And he advocates a method of using them properly. And he did actually use this method or something like it.

46
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He did put a lot of effort into natural science.

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He was the first person to explain how the rainbow works in detail.

48
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He discovered coordinate geometry. So at school, when you were learning about X and Y coordinates and they were called Cartesian coordinates.

49
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That's after Descartes. He suggested the circulation of the blood, which we normally associate with William Harvey.

50
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He concluded that the Earth orbits the sun.

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He was reluctant to publish that because Galileo, having said that the Earth orbits the sun, was condemned by the Roman Catholic Church and punished.

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So Descartes actually suppressed his work in which he was arguing for the same thing.

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But the most important intellectual legacy of Descartes is really this ideal of a mechanistic

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science of the world based on the simple mathematical properties of extended matter.

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So effectively, what he wanted to do was to say we can understand the world not in terms of Aristotelian strivings,

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but in terms of simple, inert matter, simply consisting of extended stuff spatially extended and all the real qualities of matter.

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According to Descartes, follow from that essence.

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The laws of motion, Descartes claims can be known simply by seeing what follows from the essence of matter, if matter is just extended stuff.

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Then we can see why one bit of extended stuff cannot move without pushing other extended stuff out of the way.

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If matter just as extension, then you can't have two bits of extension, as it were, overlapping.

61
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We can see why bodies are passive, why they remain in the same state and lesser force is applied because there's nothing in the nature of extension,

62
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just geometrical extended ness in space. That implies any activity.

63
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So apparently the passivity, the inertia can be explained.

64
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Of course, there are some qualities of matter that don't seem to follow directly from its nature as extension colour.

65
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For example, when we look at something and we see it as red or green or blue.

66
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Yeah. But that's actually not really a property of the matter.

67
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That is to do with the interaction between it and our visual senses.

68
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So the colour is a secondary property. It's not a real property or property of matter in a certain sense.

69
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It's due to the sensory interaction between us and the matter.

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Again, mind is quite distinct. Mind for for Descartes is completely different from matter.

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Its essence is thinking rather than extension. Since matters is extension, nonmaterial extension is impossible.

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Think about that, Descartes is saying the essence of matter.

73
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What really constitutes matter is just extended Nissin space.

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Now, that means wherever you have extended Nissin space, you must have matter.

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You cannot have an empty space devoid of matter. There can be no such thing extension, just these matter.

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It follows that the whole world is what we call a Planum. It's full up.

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There's no gaps. Okay.

78
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Now think about what that implies about motion. Suppose you've got a bit of stuff here and it's moving downwards.

79
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Well, we cannot have empty space.

80
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So if that's moving downwards, the stuff below it has to move and the stuff above it has to move down to fill into the space.

81
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Right. So it follows that all motion must inevitably take the form of a circuit, a vortex.

82
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That's the only way you can have motion within a world in which the essence of matter is extension.

83
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So there is no empty space. So it follows that everything that happens in the physical world, all the motion that happens,

84
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must be a series of vortices, whirlpools, one within another, within another, and so on.

85
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Huh. We have that problem about the planets. Why don't the planets shoot off into space?

86
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Got it. There's a vortex, a vortex around the sun and the planets are carried around in that vortex.

87
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And then there are vortices around the planets which carry their satellites around them.

88
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So the satellites of Jupiter. And there's a vortex around the earth that carries the moon around the earth and so on.

89
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So Descartes ends up with a rather nice a neat physical theory and certainly a very effective challenge to Aristotle.

90
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He has the makings of an account of the physical world,

91
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which looks much more explanatory than Aristotle's, much more amenable to mathematical calculation.

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And he's got an account of the mind which makes it quite separate from the physical world.