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Just a very quick recap from last time to remind you how we got here.

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We're looking back to the philosophy of the 17th and 18th centuries. And in these first two and a half lectures,

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I'm basically explaining why that is such an interesting and pivotal period for the development

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of philosophy and also why the problems that arose then remain very pressing today.

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So we saw that in the mediaeval world, the view of the universe was basically dominated by Aristotle and the Bible.

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And Aristotle had a physical theory which essentially explained the workings of things in the world on the model of human or animal behaviour.

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The idea was roughly that the reason why stones fall is that they're striving to reach the centre of the universe.

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They have something like a desire. Now, this came under a lot of pressure from two particular directions.

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One of them was to do with Aristotle's Theory of the Heavens.

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Stones fall because they strive to reach the centre of the universe. What about planets?

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What about the sun? What about the moon? Well, we see those moving in circles.

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They must be striving to move in circles.

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And the Aristotelian explanation was based on the idea that they must be made of a completely different stuff.

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Whose nature is completely different from that of stones on a stone strive to reach the centre of the universe.

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Therefore, the centre of the earth things planets, stars, the moon must be made of different stuff.

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Aether, which instead strives to imitate the perfection of God by moving in circles.

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That cut came under great pressure when Galileo turned his telescope towards the sky and saw various phenomena which simply contradicted it.

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The heavens proved not to be as perfect and unchanging as people had thought.

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There were craters and mountains on the moon. Not everything, sir, circuited around the earth.

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Jupiter had moons going around it and in particular, observations of Venus and the phases of Venus.

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Simply contradicted the ancient theory of how these things worked.

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Coupled with that was a realisation that the Aristotelian theory of science was seriously defective.

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If you say that such and such acts in a particular way because it strives or desires to act in that way, it it's clear, if you think about it,

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that's completely unexplored entry because that kind of theory could be used to explain anything at all, no matter how something behaves.

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You can say, oh, well, it does it that way because it wants to. Unless you've got some way of discriminating between the sorts of things,

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sorts of ways in which things can be expected to behave in the way they can't, simply appealing to intentions becomes completely empty.

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And a far better theory came along in the 17th century.

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Again, Galileo played a major part in getting it started. And Descartes was absolutely crucial in getting it established.

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The theory of mechanism. The idea that things, physical things behave as they do not because they have anything analogous to desires,

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but rather because they bump into each other, push each other either by pressure or impact.

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So we get a mechanical theory of the world, which seems much more genuinely explanatory.

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And this theory was applied in various contexts with success remained a problem about how to explain the circulation,

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for example, of the moon around the earth. How do you explain that?

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If everything is moving simply in accordance with impact from other things, and if bodies act as Galileo and Descartes thought,

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under inertia, that is, they just keep going in the same direction, at the same speed, unless they're acted upon by force.

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How do we explain why heavenly bodies orbit around other heavenly bodies?

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That remains a problem. Descartes, as we saw, got a really neat explanation of that.

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His neat explanation was that the essence of matter is extension.

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Everything fundamentally about matter can be understood by considering it as geometrically extended stuff.

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So wherever you have extension, you have matter. That means the earth is completely full.

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There is no space, no empty space.

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And it follows that all motion everywhere in the heavens and down here as well must take the form of circuits or vortices.

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If they cannot the empty space, then that's the only way things can move by something else,

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taking its place and pushing whatever is in the place where it moves to round in the vortex.

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So the explanation of the orbiting of the planets, the orbiting of the moon and so on is in terms of these vortices.

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All seems very neat.