So you can see this is in many ways a very modern vision. There is no surprise that Hume, out of all the philosophers of this period, is the philosopher who tends to speak most to the modern predicament. We live in a world in which people do think that the right way to understand human beings is as part of the natural world. Looking back to the 17th and 18th centuries, it's vitally important when you read the philosophy, the philosophers of that period, not to forget the elephant in the room.
God. Almost every philosophical discussion of the period is informed by religious concerns. They're often not overt. It's often not easy to see them. But behind it all lies these fundamental changes. An earthquake that's going on in the foundations of the view of man and the universe and. The threat of Athie ism, the threat of heterodoxy. That lies behind a lot of the concerns that the philosophers of the period have. So here's a very simplistic big picture.
You go back to the mediaeval period, physics is governed by natural motions, natural motions that have been put into things by God. God has created stone so that they naturally tend towards the centre of the world planet, which naturally tend to want to move in circles. That's replaced in the early modern period by a picture of inert matter, mechanical causation. Things do what they do because of forces acting on them rather than anything like desire's acting within them.
Morality in the mediaeval period is based on revealed truth and natural law. We can go to the Bible. We can apply our reason. To that to the lessons that we learnt from that. But in the early modern period, that consensus gets undermined. We now no longer whether know whether we should seek authority in the church traditions or in the in the individual's reaction to the Bible. So. Can we base morality on Revelation when people interpret the revelation in different ways?
Can reason fill the void? Should we, like Hume, rely on moral sense or feeling? Likewise, in politics, in the mediaeval world, we have the divinely ordained king who should be obeyed for that reason. Confidence in that authority is undermined, not least through the religious wars. And so we get a crisis of authority in politics. How is that to be filled? An appeal to natural right reason. Contract.
Hobbs wanted to say that the way to do it is all to get together and make a social contract, erect a sovereign and then obey the sovereign. In order to avoid civil war. So you can see that the picture I gave in the first lecture of the crisis that affected the whole intellectual world in the early modern period, we can see that those of problems that echo down to this day.
And this is why so many of the problems that we discuss within this course remain of relevance, even though the scientific theories that we have now seem to be a long way away from those that were around in the early modern period for us, just like them. It seems that the world differs radically from how it appears. Our best theory of the world attributes primary qualities to bodies with secondary qualities explained through a representative theory of perception.
As we've seen in the early modern period, the idea that matter is made of little corpuscles, very different from what we see. We may see something as a particular colour. We may see a tree as brown. But actually, what's really there, we think, is little corpuscles whose arrangement gives rise to the sensation of Brown in our eyes, the reality and what we see are very different. Okay. Our modern theory attributes different properties to the actual matter of the tree.
But the same problem is there. And this invites scepticism. If we can't trust our natural faculties to yield truth directly, we can't trust them to show us how things really are. Then how can we know how they are? Again, if the actions of body are explained mechanically, how does mind? As science grows in the early modern period through a purely mechanistic science. Now we have more sophisticated stuff like quantum mechanics. OK.
It's not purely deterministic in the way that those early theories were. But it yields a very similar problem. If we think of ourselves as part of the natural world as constituted by material bodies and brain and so forth, then how does the action of that physical matter tie in with our view of our minds? And in particular, free will. How can we be free if all of our actions are the actions of bodies which are themselves determined or at least condition by physical laws,
can free will actually make it make any sense? And if one believes in immortality and divine reward and punishment, how can that make any sense? That's a particular problem because of personal identity. If we think of ourselves as constituted as part of nature by material bodies and brains and so forth, does personal identity over time become something that's applicable at all, particularly in the context of religion?
Well. We shall see that these sorts of debates echo through those topics in general philosophy, and I hope that when we study those in a bit more detail, you will see how they tie together. All as part of this crisis, which is very much the legacy of the early modern period. Okay, I'm just going to end with a brief comment about what happened after the early modern period. Well, we saw Hume leaving us with a rather. Unsettling picture of human nature.
Humans. Part of the animal world. Not nearly as clever as they thought. They were reliant on brute animal instinct to find out about the world. Quite incapable of knowing about things. By pure reason. Then along came Emmanuel Kant. Very famous philosopher. Kempt started from the premise that Hume has to be wrong. Why does Hume have to be wrong? Well, Camp thought that there are certain things that we do know about the world with absolute and complete certainty.
Here is some of them. We know with certainty, according to Kent, that the world has to be governed by universal causation. We know, according to Kent, that the principles of Euclidian geometry are utterly and completely certain. For example, the square on the hypotenuse of a right angle triangle is equal to the sum of the squares of the other two sides. We can prove it. We can prove it by pure logic. That is a truth about the world that we know with absolute certainty.
And what about Newtonian mechanics? For example, the law of conservation of momentum? That has such a natural, elegant simplicity to it. According to Kant, this, again, is a principle that we can know to be true a priori eye of the world. We can know simply by applying our pure reason that these things are all true of the world. It follows that Hume must be wrong because if Hume is right, then it isn't possible to apply our pure reason to know things about the world.
Kent developed a very elaborate theory to explain how it was that Hume could be wrong, according to Kent. Our minds condition the way the world appears to us. And so we can know a priori by how the world will appear. The phenomenal world that is the world that we experience must, for example, satisfy the axioms of Euclidean geometry because our minds themselves constitute it in such a way. Very interesting theory. Unfortunately, its premises are completely wrong.
So let's look at what happened after Kent. Darwin's on the Origin of Species, 1859. A stronger confirmation, as one could wish. That we are indeed part of nature, not above it. Einstein's Theory of General Relativity, 1915. Space, it seems, is gravitationally curved. Euclid's axioms probably aren't true of the actual world after all. At any rate, they're certainly not knowable a priori, I can turn all these others had assumed that geometry does give us pure insight into the way the world is.
It seems that that is not the case. The logical deductions that we make from the axioms may be fine if the axioms are true of the world, but we've no way of knowing a priori that they are true of the world. And then we get quantum mechanics in 1925. Or thereabouts. It was quite some long development. Undermining the idea that the world is law governed in the way that Kant thought and severely undermining the idea that it is intelligible.
And I'm just going to give you a brief illustration of this. So this is a computer model of the famous two slit experiment. Here you have a light source at the bottom. And here you have a screen with two slips in it, very small slits. The light travels through these slits. And then at the far end here we have a screen on which we see where the light has fallen. And in what intensity and what you can see here is that you get this interesting pattern.
Why does that pattern occur? Well, it seems that light has a wave form. So if I do this, you can now see how you're getting an interference pattern. You've got waves going out from each of the slits and where they meet, they interfere with each other. Just like ripples on a pond. If you drop two stones into a pond and you get the ripples coming from each of them, wherever the ripples coincide, they're both high or they're both low.
The combination of the two will be even higher or lower. But at other points, you'll get an upward ripple, combining with a downward ripple. And the two will cancel out. So you get an interference pattern. Light. It seems, is constituted by waves. All well and good, but. If that pattern was a result of interference, then presumably what we can do is get rid of the interference by firing single particles of light single photons at the screen.
So what I've done now, I've put a detector on the left slit and a detector at the right slits, and I'm going to fire individual photons at the screen. Let's do that. So I want to show these on the screen. So here we are. I'm firing them. You can see that the photons are going randomly through the two detectors and then they're ending up randomly on the screen. Now, let's speed that up and see what happens.
So what we now have is individual photons going to the screen, the ones that get through either go through the left slit or the right slit. Then they go on to hit the screen at the back. And you can see that the interference pattern has completely disappeared. Fine. All nice, straightforward, we understand pretty much what's going on. But let's try that again. Except this time I'll take the detector's away. So now I'm firing individual photons at the screen, as you see. Let's do it repeatedly.
How weird. What on earth is going on? If we fire the individual photons and have detectors at the slips to find out which way each photon went. The interference pattern disappears. If we then take the detectors away, we're still firing individual photons one at a time, but we no longer know which slip they're going through. Somehow the photons still end up ending up on the screen in an interference pattern. How can that possibly be? How can there being two slits rather than one?
Make a difference to where the photon goes? If when you put a detector there, you only ever find the detector going through one slip rather than the other. It's seriously weird. Seriously, seriously weird. Now you can do the mathematics to find out what's going on. You can show that if you put a detector on, either slit the wave, the wave probability function changes. But that's not explaining why it happens. It's just saying this is the way it does happen.
And I think quantum mechanics is a beautiful example of how humans approach to science has turned out to be right rather than Kentz. It seemed when Newton came out with the beautiful mathematics of his prankish here. But we were getting real insight into the way the will the way the world works and why it works that way. It all seemed to be so logical. And yet, as modern science has gone on, we found that trying to understand why it works as it does.
Is a dead end. We have to make do with codifying how it works. Not why does it? OK. Incidentally, if you want to find out more about the stuff I'd been talking about in these first two and a half lectures. You might be interested to look at the introduction to my edition of Hume's enquiry, in which I give quite a lots of more detail on on all of this stuff. I didn't know how that little a little subliminal bit got in.