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Okay, so you can see why there was an enormous amount of doubt that this time and very good reason for it.

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The traditional world was crumbling in all sorts of ways. There were all sorts of horrible tensions in there leading to bloody walls.

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And at the centre of those wars was an intractable dispute, which seemed very, very pointedly to raise a fundamental philosophical problem.

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How can we know what's right? Well, very soon things started happening in the realm of science and let's just see how that came about,

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Aristotelian science was based on a very commonsensical idea, actually, an idea Aristotle had got from the pre Socratic philosopher Empedocles.

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There are four elements fire, water and Earth.

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Everything we see is made up of this mixtures of these basic four elements.

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Now, these things have natural motions. For example, if you take something that's made primarily of Earth and you let go of it, it falls.

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We're used to that. Earth has a natural motion downwards.

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Fire has a natural motion upwards. Well, this all makes sense.

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Things made of earth are seeking their natural place. The natural place of earth material made of earth is the centre of the universe.

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The centre of the Earth. So it's entirely natural that things made of earth moved towards that.

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Likewise, fire, air, water. Their natural position is with Earth in a sphere at the centre, then water, then air, then fire.

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And the motions we see match with that. The heavier things are, the more earth they contain, the faster they fall in proportion to their weight.

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What about the heavens? We look up at the heavens and we see bodies that do not fall towards the earth.

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Instead, they move in circles around the earth. How can that be?

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Well, they must be made of a different kind of stuff. Let's call it aether.

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So there is this fifth element, this quintessence, the fifth essence of which all the heavenly bodies are composed.

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That explains why they move in circles. That is their natural motion.

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Why should circularity be their natural motion? Well, circularity is the closest they can get to the perfection of the eternal God.

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So the explanation for why things work as they do in nature is all in terms of things striving to achieve a particular end.

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A body is made of earth, strive to reach the centre of the universe.

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Bodies in the heavens strive to move in the perfection of circles.

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Now, this came to be seen as a bit problematic. So suppose, for example, we take a siphon here, we've got a siphon.

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I'm sure this is familiar to all of you on the left. You've got a beaker with water in it and the pipe is going into that.

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And the pipe itself is full of water. And as we know,

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what will happen is that the water will pass up the pipe and down on the right hand side and the

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beaker on the left will be emptied and the beaker on the right will fill up correspondingly.

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Now, why does the water on the left rise up the pipe?

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It's easy to understand why the water on the right falls down. Okay. That's trying to reach its natural position.

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So it moves downwards. We know water moves downwards when it's when it's left to itself, as in the rain.

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But you see, if it moved downwards on the right, that would leave a gap, a vacuum, a hole in the pipe.

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It would leave an emptiness. So why does water move up on the left hand side?

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Well, it must be that nature abhors that emptiness. Nature abhors a vacuum.

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So the water on the left moves up in order to avoid there being a vacuum.

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It's striving to avoid this have Horan's situation of a vacuum.

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And it's actually a very natural way to think, isn't it? I mean, think of sucking water up a straw.

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You think of yourself as sucking it up. The water is coming up to fill the void that we know now.

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That isn't what's going on. Rather, it's atmospheric pressure pushing.

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But that was the very natural explanation that they came to.

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But compare this explanation, and this is actually from a a a parody Molad Imagineer, a play of Moliere in 16 73.

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And he's making fun of this kind of supposed scientific explanation.

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So somebody asked the doctor, why does opium make one's sleep?

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And the answer comes because it contains a dormitory virtue whose nature is to make the senses soporific.

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Now, clearly, it's a parody. There's no explanation at all.

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Why does it make you sleep? Oh, because it contains something that makes you sleep.

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And if you think about it, the same is true of the explanation with the vacuum.

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Nature abhors a vacuum doesn't explain anything. All it says is that vacuums won't naturally occur.

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It doesn't say why. And now think about stones striving to reach the centre of the earth.

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Why do stones fall? Because they strive to reach the centre of the Earth. That doesn't actually tell you anything about why it's happening.

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It's just read describing the phenomenon. It's just saying in effect.

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Well, they do. Now, not only was Aristotelian science very unsatisfactory from this point of view,

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simply saying that something does X because it wants to do X doesn't actually give you any explanation at all.

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But the worst specific problems with it, take the flight of a cannonball.

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How does a cannonball fly? Well, roughly, it's a parabola.

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The shape of the curve when it falls is almost the same as the shape of the curve when it's going up into the air.

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Slight difference because of air resistance. It'll slow down a bit, but more or less it's a parabola.

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But you go and look at pictures at the time before the Earth, just for the early modern period.

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Look at pictures of Cannonball Flight and you'll see they describe it like this as though at the end of the flight, it's dropping almost vertically.

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And that's because their theory of motion would imply that while there's an initial impetus from the explosion,

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sure, the cannonball can keep going horizontally. But as soon as that impetus goes, it just restores its natural motion, which is downwards.

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We'll take a sledge sliding on flat ice. I suppose you've got a lake of water that's frozen over.

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So very smooth, very flat ice. And you push a sledge on it and then you let go.

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What happens? The sledge keeps going. How can that be?

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Once you've stopped pushing. How is it that the sledge keeps going?

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Well, Aristotelian came up with various explanations.

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They said, oh, well, as it goes through the air, it sets up vortices of air and the vortices keep pushing it.

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That's why it keeps going. None of these explanations was very satisfactory.

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What Galileo said was, no, actually, the sledge keeps going because the natural motion of things is to keep going in the same direction.

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They're going at the same speed unless acted upon by force. What requires explanation is not why the sledge keeps going.

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What requires explanation is weight stops. And that's a fundamentally different view of things.

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No longer are you saying that things naturally move towards the centre of the earth.

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You're now saying that they simply stay where they are or keep going in a uniform direction, a uniform speed, unless they are acted upon by force.

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And that, of course, introduces forces like gravity. Galileo was famously reported to have performed another experiment.

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Scholars doubt, actually, whether he really did. But it's a nice story.

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Go to the top of the Leaning Tower of Pisa. Take a say, a cannonball on a marble large and a small ball.

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Drop them and see how fast they fall.

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And what you will find is that they fall at almost the same speed, not exactly the same speed because of air resistance.

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But you say maybe you you will have seen on the web or whatever.

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I remember vividly from when I was a lad, when David Scott, the Apollo astronaut, I think it was Apollo 17 or something, went to the moon,

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took out his hammer, took out a feather, dropped them on the moon, where,

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of course, there is no atmosphere, and they fell at the same speed round together.

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A very nice illustration that Aristotle is wrong on Aristotle's theory.

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The larger object should fall much, much faster than the smaller object.

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So Aristotle's physics is looking in serious trouble for a range of reasons.

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But that's not the worst. That was to happen. The telescope was invented in sixteen oh eight.

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Its initial uses were things like military. You can see when the enemy is advancing, you can see how many there are and so on.

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Or if you're a merchant and you want to know whether your ship is arriving into port,

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you can take a look and see things at much greater distance than before.

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What Galileo did, as well as perfecting the telescope, he made a much better one than anyone had before.

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He had the bright idea of turning it upwards, looking at the sky.

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Well, what did he find? He found mountains and valleys on the moon, even found by looking at them at particular times of day.

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He could see the shadows cast by these by the sun and even estimate their height.

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He found four moons orbiting around Jupiter. Of course, we now know there are far more.

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But before that, he saw with his telescope very famous.

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He chose. Not everything is centring around the innumerable stars.

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I mean, when you look through a telescope at, say,

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the Milky Way and of course the Milky Way was much more visible for him than it is for us today with all the street lighting that we get.

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Look at the Milky Way through a telescope and you find that behind, as it were.

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All the stars that we can see are zillions more the idea that the stars are on some fixed crystalline sphere that rotates around the world.

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Once you look at that kind of depth becomes less plausible.

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And he saw that Venus has phases. Now we're used to the moon having phases.

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Sometimes it appears as a crescent. Sometimes it doesn't appear at all.

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It's just black. Sometimes we see the full moon.

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Well, the same is true of Venus. Now, the Aristotelian theory, as I said, was based on the idea that all heavenly bodies basically move in circles.

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Now, that's a bit of a simplification.

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If you look at the motion of the planets in the sky, you will find that they don't simply go round the Earth in a continuous circle.

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Sometimes they seem to move backwards for a time and then move forwards again.

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Now we understand that, of course, because we think of the Earth as orbiting around the Sun and say Mars also orbits around the sun.

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And so we've got a complication of two different motions added together,

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which means that in general, Mars will move in a consistent direction around the Earth.

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But sometimes, as it were, the earth overtakes. It's a Mars seems to go backwards.

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So how did the Aristotelian deal with that?

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I mean, how did they explain the fact that the motion of the planets visibly was not just a straight, consistent circle?

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Well, they explained it in terms of planets moving on circles around circles.

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So instead of simply having a circle like that,

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you might have an arrangement like this where planets is moving on one circle that goes around another circle.

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And by doing that, they were able to give an account of the position of the planets in the sky.

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That worked out pretty well of the order of five degrees plus or minus.

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So not great, but enough to keep them relatively content.

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Now, in order to explain the motion of Venus in the sky, there's a bit more of a problem.

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You see, Venus, as we know now, is closer to the sun than the Earth is.

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Its orbit is within the earth's orbit.

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So that means that Venus never gets more than a certain angle away from the angle of the sun, which is why Venus is mourning star and evening star.

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Okay. It only appears in ours fairly close to the sun's appearing.

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So how do you explain that within an Aristotelian model? Well, the way it was done was developed by an Alexandrine called Ptolemy, very famous.

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He had the sun moving around the Earth and Venus orbiting around a point on the line between the Earth and the Sun.

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Okay, so Venus went like that. Here's the Earth.

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The sun is going round and Venus is going round like this, always roughly between the Earth and the sun.

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Here's the problem. Imagine what do you see from the earth?

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When you look at Venus, when Venus is illuminated by the sun like that, what you see is a crescent.

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Agreed. You never see a full Venus.

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You cannot see a fully illuminated Venus. You can only see it fully, fully illuminated Venus.

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If Venus is sometimes on the other side of the sun, even then, of course,

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you never see fully limit illuminated because that would the sun would be in the way, right.

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The sun is far too bright. But the point is that what we do see is Venus nearly full.

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When the sun has just gone down, there's Venus nearly full.

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And the only possible conclusion is that Venus is on the other side of the sun from us at that point.

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So these precise Arista t is sorry, a strong astronomical observations really caused immense problem for the Aristotelian world view.

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They blow it apart. Okay, so that helps to explain why Galileo was such an important figure, had a great influence on Descartes as well.

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And as I've said, it's not just the detailed astronomy that that is the problem.

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It's the whole conception of science. Aristotelian science was based on purposes, what's called final causation thing,

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striving to reach a particular final state, whereas Galileo preferred a model based on efficient causation.

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One thing bashing into another, making it move a matter doesn't strive.

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It's not the material. Things have some sort of desire to reach a particular state.

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Instead, things happen because one thing acts on.

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Another pushes it as in billiard balls banging into another, or as in water in a pipe pushing it along.

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So the outcome doesn't depend on some foreseen final state.

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It depends on where the causal sequence happens to lead. And so you get the ideal of the mechanical philosophy, the paradigm,

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the sort of standard thought about efficient causation is mechanical contact that is intelligible.

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The Aristotelian is had said that things move because they're striving to reach a particular situation.

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They thought that was intelligible.

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They thought that made the world comprehensible because it made physical things act in the same sort of way as we do.

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But the advocates of the mechanical philosophy, the early modern period, wanted to say,

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no, that isn't the right kind of intelligibly ability that we should be looking for.

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We should understand causation in terms of one thing bashing into another,

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the familiar contact of billiard balls or as I said, pressure of water in a pipe.

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That seems both genuinely explanatory and genuinely intelligible.