▶︎ 0:01 So now we can begin the big argument. This is the core of the book, the center of the book. It's an amazing argument. It is difficult, but it's really extraordinary in its conclusion. It's come to be called the problem of induction. He doesn't call it that, and I'm not going to get into any real details about what that means, but you might hear it referred to as that.
▶︎ 0:25 So the question that he has is, how do we reason about matters of fact? Reason about relations of ideas is simple. There's nothing else it could be. It can only be that one answer. Anything else is a contradiction, and that's why it's necessary as well as uninformative. But matters of fact, much, much more important, much more numerous, how do we reason about those?
▶︎ 0:48 Well, there are some matters of fact we have an immediate impression of. We can directly see. How do I know that my shirt is blue? Just because I see it. Again, uninteresting, obvious, no big deal. But again, that's a tiny, tiny fraction of the beliefs that I have.
▶︎ 1:09 I have tons of beliefs about matters of fact that I have no immediate perception of, that are outside of this room, outside of my empirical data that I'm having at the present moment. How do I know those? How do I reason towards them? How do I have justified beliefs about those? This is the question that he's asking now, and he's going to go through a series of steps to get there.
▶︎ 1:44 And he's going to make one surprising claim and one shocking claim. So we're going to have to see how those work and if we can let him get away with those, and if we can, if you can have any objection to them.
▶︎ 2:01 His first question is: How do we know matters of fact, MOF, that we do not have immediate, direct impressions to tell us what they are? And his answer to that is cause and effect, a causal relationship. His example, he gives the example of Robinson Crusoe, a novel that was very popular at the time, of this guy stuck on a desert island, a deserted island, and he thinks he's alone. And then one day, he realizes there's someone else on the island, not because he sees them. That would be a direct impression. His reasoning that there's another person by seeing them is not very interesting.
▶︎ 2:45 Instead, he sees a footprint on the beach where he hadn't been. So he sees a footprint and he goes, "Aha! Another person exists here." Hume says, "What's the reasoning that happens?" He doesn't have a direct perception. It's contingent. Could be another person, might not be, so it's a matter of fact.
▶︎ 3:08 How does he do it? Because the footprint is the effect that must be caused by a human foot. He is taking the effect, which is an immediate impression, which is something he's got a direct experience of, and he's using the causal relationship to get outside of his immediate experience. He doesn't need to reason that there's a footprint. He sees it. But he does need to reason to what he does not see, which is the other person.
▶︎ 3:43 And the way he moves from what he does see to what he does not see is through a causal relationship. Something must have caused this footprint as an effect. It must be something that could account for that, and the only thing that does is a human foot. Therefore, another person exists on this island. So cause and effect is the way that we build up our matters of fact, knowledge about the world beyond simply what we can hear, smell, touch, or taste at the moment.
▶︎ 4:18 And this is what science is. Science is what causes what. Laws are just laws of regular causal relationships. That's what science is. Science is about bringing phenomena into causal relationships. So if you want to understand science, this is what Descartes wanted, you have to understand causality. And of course, for Hume, causality is contingent and empirical.
▶︎ 4:47 Now, of course, we have an answer, but it's not a complete answer, because now we can ask the same question. How do we know this? How do we reason about cause and effect? How do we link causes and effects together in an intelligible, rational, justified way? That's his new question. Got his first question, first answer. Now he has a second question about that answer.
▶︎ 5:27 And here we get to the first surprising claim. So imagine that you're in a pool hall, and you see one billiard ball coming towards another one and striking the second one. Now, what do you expect the second one to do? You're going to expect the second one to move. First one hits it, the second one goes in a direction. Hume says, "How did you know that? Where did that knowledge come from?"
▶︎ 6:04 Now, what it feels like is, well, duh, obviously what else could it do? The second one's going to move. If the ball hits the other ball, the second ball's going to take off. This is logical, this is rational, I can just simply infer this. I can logically infer what will happen when one physical object hits another one. And Hume says, "No, you can't."
▶︎ 6:36 The thought experiment that he runs, and this is just a thought experiment, nothing religious or anything, is we bring Adam, from Adam and Eve, into the pool hall. This isn't the beginning of a joke. We bring Adam in, and again, the first billiard ball goes towards the second one and is about to strike it, and we freeze time. Because what the hell, with Adam, we can do whatever we want. We freeze time just before it hits the second one. And we turn to Adam, say, "Okay, Adam, what's going to happen?"
▶︎ 7:12 Adam is brand new created. Now the reason why this is a great thought experiment is because Adam has complete faculties; intelligent, rational, can reason, but no experience. He has seen nothing of the world. And we ask Adam, "What's going to happen to the second billiard ball when the first one hits it?" And Hume says, "Adam won't have a clue." Have no idea.
▶︎ 7:44 He's never seen this before. He doesn't know what happens when physical objects bang into each other. Now, to you and I, we've seen this happen, I can't even put a number to how many times we've seen physical objects hit each other, so of course we expect the second one to move, of course we know that's what's gonna happen. And we've seen it so many times that it seems so obvious and seems rational. It seems inevitable. Seems like it's the only thing that could happen, and of course we could just think it out and we could expect that.
▶︎ 8:16 But if you think about it for a minute, if you think about what would happen if you'd never seen it before, anything could happen. The second ball could just be completely still. The ball could go straight up. The ball could explode, it could grow legs and dance and sing. It could do anything. You have no way of knowing what it would do if you have not had experience, if you've never seen what happened. Adam wouldn't have the faintest idea.
▶︎ 8:54 So it feels to us like we could reason out causal relationships because they're so familiar, but Hume says, "Reason plays no role in learning what causes what, only experience." Only experience. If you have not had experience of a cause and effect, you cannot know it. He says, "Adam could not know that water would drown him and a cloud wouldn't."
▶︎ 9:33 You couldn't know quartz. Quartz is a crystal. The word crystal comes from the ancient Greek for icy cold, 'cause it looks like ice. It makes up 12% of Earth's land surface, 20% of its crust. And you know how it's created? In lava. Lava makes crystal. Lava makes quartz. I never would've known that. I never would've expected. It looks like ice, looks cold. But only because people have seen it will they know that it's extreme heat that causes crystal and not extreme cold.
▶︎ 10:17 These causal relations only feel like we could reason them out because they're so familiar, because we've seen them so many times. But in fact, the only way we know them is through experience and through repeat experience. You take Adam to the pool room the first time, you freeze time, you say, "Stop. What's gonna happen?" He says, "I have no idea." That's the first surprising claim.
▶︎ 10:50 Now, we've answered the first question, how do we know matters of fact that aren't immediately apparent? Through cause and effect. How do we know cause and effect, second question? Only through experience.
▶︎ 11:05 What's the next question? How do we use experience to understand causal relationships? And this is going to be his shocking claim, because we take Adam in to the pool hall. The first time we ask him what's going to happen, he has no idea. We leave him there, come back two hours later. Hopefully he's not drunk, lost all his money. We do the same experiment.
▶︎ 11:36 The first billiard ball is about to strike the second one, and we freeze time. He's been there for two hours now. He's been watching pool games, he's been watching billiard balls hit other billiard balls for two hours, and we ask him, "Now what's going to happen?" He says, "Oh, the second ball's going to move." He knows it easily. No problem. Of course, the second one's going to move. Now he knows it. Now he can make the inference from the cause, the first billiard ball hitting the second one, to the effect, the second one moving. Why? Because he's had experience.
▶︎ 12:12 Hume's question is, how does that work? What is this arrow? How does experience underwrite it? And his answer, his shocking answer is, it's not a reasoning inference. In fact, there is no reason for Adam to expect the second billiard ball to move at the end of two hours. There's no more reason then than at the beginning. There's no reason for you to expect the second billiard ball to move. If you think the second ball will move, you are not doing so on a rational basis.
▶︎ 13:03 That's a shocking claim. First one is, the first time you saw something you couldn't reason it out without experience. The second claim is even after experience, after experience, after experience, years of experience, decades of experience, you still cannot reason that out. You still cannot infer an effect from a cause, no matter how many times you've seen it. This is a shocking claim.
▶︎ 13:42 Here's his argument. I am basing my expectation of what effect will arise from the cause in this moment from all of my experience, from all the times I've seen the second billiard ball move, over, and over, and over, and over again. Now, Hume says, let's slow way, way, way down. What exactly does that experience tell you about? What is that actually about? What that tells you about is all of those past moments. In those moments, the second ball moved. Yes. No denying it. Not disagreeing.
▶︎ 14:37 All those times a ball hit another ball, the second one moved. But that is all that the experience tells you. You are saying something that goes beyond that. You are not saying the ball has moved many times. He'll grant you that. You are saying now, in this moment, this ball will move. And Hume says there's a gap there. There's an explanatory space that you've got to fill. There's a space between all these times, thousands, millions of times the effect followed from the cause to, and now it will happen again.
▶︎ 15:29 It's not a relation of ideas, you see. The idea that whatever causal relation we have that has gone on for a long, long, long time, the idea that that will continue is not logically necessary. It's contingent. At any moment, the laws of physics could change. That's perfectly intelligible. There's no logical contradiction in that possibility.
▶︎ 16:10 Hume doesn't think it's going to happen. He's not saying, "Watch out." Go live today because the gravity's going to be reversed tomorrow. He's saying it's logically possible, and therefore, it's not absolutely necessary that the same laws that made the ball move all those times in the past are going to be in effect now. It's not necessary. It's not a relation of ideas. It's a matter of fact. It is contingent.
▶︎ 16:45 The idea that the laws that governed all these events in the past, the idea that those will continue the same, and therefore you could expect the same effect from the same cause, is a contingent claim. And therefore, we have to ask, how do you know? How do you know? If it could be false, then you have to have a reason to believe it's true. And Hume says you don't. There is no reason to believe that the laws of nature will continue the same way they have for billions of years, that they all could be the same way in the next five seconds.
▶︎ 17:37 There's no reason. He's not saying he doesn't think it'll happen. He's not saying, "Watch out, prepare." He's saying there's no reason for that assumption. He believes it. You believe it. I believe it. But we can't prove it.
▶︎ 17:57 We can't even have a reason for it. And here's why, and this is very hard to understand. This is a very tricky argument. If you don't get it at first, then you're not stupid. It's hard to get. What he's saying is, in order for the past experience to be predictive of the future, which is what we're saying, we're saying that the future's the next few seconds, the second billiard ball moving, you have to prove something. You have to prove that the future will resemble the past, that the basic laws of nature governing these actions will maintain in a stable fashion, will continue the same way they've been. And we can call this the future will resemble the past.
▶︎ 18:54 This is the idea you need. If the future might not resemble the past, then all of this past experience is useless for predicting what's gonna happen. It means nothing. Because if things change, then this tells you nothing of what's about to happen. It only tells you what's gonna happen if things remain basically the same, if the framework, the laws of physics, the laws of nature, remain basically the same. So you have to prove this, and you can't.
▶︎ 19:30 It's not a relation by ideas, it's contingent. And if it's a matter of fact, well how do we determine that? It's not something that I have a perception of, I don't have an impression of it, it's in the future. Can't have an impression of the future. Instead, it's gotta be the same kind of matter of fact that we have. Well, how do we prove these matters of fact? From past experience.
▶︎ 20:03 Why do you believe that the laws of nature will remain stable? 'Cause they always have. There's never been a moment where the law of gravity suddenly reverses. There's never been a moment where, all of a sudden, the billiard balls start getting up and dancing. There's never been a purge moment for the laws of gravity, the way the purge is for the laws of society if you get a murder once a year or something. I haven't seen those movies, something like that. They've never changed before, so why would they now?
▶︎ 20:36 Well, that's great, except you are using past experience to prove this, to prove that the future will resemble the past, but the only way you can use past experience is if you assume the future will resemble the past. You have to assume what you are trying to prove in order to prove it, and that's a logical fallacy. That, by the way, is what begging the question means, not bringing up a question. That's a little irritation to me. I'm sorry, it's a pet peeve. You don't beg the question. If that begs the question, no, that raises the question. Begging the question is a logical fallacy where you assume what you're meant to prove.
▶︎ 21:26 We would naturally use the past experience of the stability of nature to predict that nature will continue being stable. It's always been stable, it's never changed, so it's going to continue to be stable. Well, that's great, but if there's the possibility of it changing, which there is, it's contingent, then all that past experience of the stability doesn't predict its future stability for the same exact reason. The same exact reason. In order to use the past experience of the stability of nature, you have to know that nature will be stable, but that's what we don't know. That's what we're trying to prove. So we can't prove it.
▶︎ 22:23 There's a turkey, lived on a farm, and the farmer's a wonderful man. Farmer's a lovely man, every day he comes out and he feeds us this wonderful food. He's terrific, love that guy. Every day of my life, he's a wonderful, wonderful man, until Thanksgiving. And then he comes out and he grabs the turkey and kills him.
▶︎ 22:50 Something changed that the turkey was not privy to. The turkey didn't know the holiday season was upon us. The turkey doesn't know the calendar. What Hume is saying is, for all we know, we're the turkey and there are some forces beyond us that could change without our being aware of it.
▶︎ 23:14 The turkey assumed the future resembled the past. Every day, the farmer's been wonderful, but then the future stopped resembling the past. The future changed due to forces that the turkey didn't know about. Hume is saying we cannot rule out the idea that we're the turkey and there's some cosmic Thanksgiving out there.
▶︎ 23:39 Not because he believes it, not because he's warning us, "Watch out," but he's trying to explain, he's trying to show us something about human understanding. He's trying to show us that matters of fact, the matters of fact that go beyond our present experience are the most important things there are for us to know how to live, and there's nothing supporting them.
▶︎ 24:18 There's no reason, there cannot be. You have no reason to expect that eating bread tomorrow will be more nutritious for you than eating glass will be harmful to you. In the past, they always have. In the past, eating glass is bad and eating bread is good. Will it be tomorrow? You can't know.
▶︎ 24:42 In order to know that, you'd have to know that bread would maintain the same properties, glass would maintain the same properties, your stomach would maintain the same properties, and you can't know that, because the only way you could prove that is by pointing to how long they have, and that past experience is irrelevant unless you know that they will continue, which is what you're trying to prove. So you can't know whether you should eat glass or not tomorrow. You can't know if you should get out of the way of cars tomorrow, five minutes from now. You can't know anything. He seems to be leaving us in epistemological nihilism. But he won't.
▶︎ 25:31 Now, I've taught Hume for many years, and just because of the way the schedule works, I usually end a class with telling people they can't know if they should eat glass or bread, and so far, I've never had an emergency call, never had a student who ate glass, and I've never worried about it. I've never been concerned because of Hume's next point. Hume argues, this argument is airtight.
▶︎ 26:11 He believes, you're of course welcome to find objections to it, please do, he believes there is no response to it, that this proves you have no reason to expect the causal relations you know and rely on to continue. But you will. That's because you cannot be reasoned out of a position that you were not reasoned into. If you follow this, if you believe, if you agree with this argument, which he thinks you should, then you will come to the conclusion that you have no rational basis for expecting all the things you know about the world to continue, but you will still expect them to continue, because they were never on a rational basis to begin with.
▶︎ 27:12 Hume argues that the reason why we believe, why Adam now after two hours in the billiard hall believes the second one will move is not because of a thought process. It's not because he has said, "Oh, this happened so many times, it must continue." It's because of a psychological feature of human nature. He calls this custom or habit. This is a fact about the way our brains are put together.
▶︎ 27:57 He is doing proto-psychology. Psychology didn't become its own discipline until the end of the 19th century basically, like Breuer and then Charcot and Freud. Up until then, if you want to do psychology, you did philosophy. It was part of philosophy. And Hume is exploring a branch of philosophy, a branch of psychology that became called behaviorism.
▶︎ 28:26 B.F. Skinner is probably its most famous practitioner. It was created by Pavlov. Pavlov was a Russian psychologist, very famous for his experiments with dogs. He had a lot of dogs, and what he would do is he would ring a bell and then feed them. Now, the first time he did this, it meant nothing to the dogs. Bell ringing, "Ugh," nothing. But he did it over and over and over again. And eventually when he rang the bell, the dogs said, "Ooh, supper." And eventually the dogs started salivating just at the sound of the bell.
▶︎ 29:07 Now, there is no dinner bell in nature. There is no natural organic reason for dogs to associate that sound with anything having to do with eating. What Pavlov did was forge a connection. He created a cause and effect relationship that was entirely artificial. It was an artifact. He manufactured it simply by cause, by first one then the other, first one then the other, first one then the other, over and over and over again.
▶︎ 29:50 And his conclusion was if you have what Hume called constant conjunction, meaning two things, when one happens, the other one does, they're conjoined consistently and in the same order, after a while, when you expose the animal to the first one, they will expect the second one. It doesn't matter how weird or how artificial, how meaningless this conjunction is, they will expect it. And so, the dogs came to expect food from a bell.
▶︎ 30:37 And B.F. Skinner had this very famous demonstration where he would give speeches and he'd have a pigeon that he'd never met before, and he'd bring the pigeon on the stage. And the pigeon would just turn left and turn right and move around in this random order. But every time the pigeon turned left, he would reward him with a pellet of food. And he would turn left, turn right, turn left, get food. Turn right, turn left, food. And within a minute of only rewarding the left turning and not the right turning, within a minute, he could train the pigeon to turn all the way around in a 360 circle. You can see it on YouTube, it's really fascinating. Because he forged this link between the consequent junction of turning left and getting the food. And this is what Hume is arguing.
▶︎ 31:34 Descartes says we are thinking things, operating on logic. Hume says we're hairless apes responding to stimuli. We are animals. We are continuous with the rest of nature. We are not this exception, we're not these limited angels, we're just clever apes. And the same things that happen to animals happen to us.
▶︎ 32:08 Yes, we're very, very smart, and we can get incredibly good at finding precisely the consequent conjunctions, but that's all that we can do. All that science is, is finding the consequent conjunctions, first this and then this. All that science is, is finding, simplifying these, finding the most universal causes. But why gravity is at 9.8 seconds per meter squared instead of 9.7, instead of 10, instead of 50? It just is. Why atoms have those many particles and not one more, not one less?
▶︎ 32:56 Science, and this isn't philosophers, scientists will say this all the time, they say, "We don't know why the fundamental forces are like that, we just know that they are. We're just studying empirically how the universe works, but the ultimate reasons why it works like this, we'll never know, because all we can do is trace how things occur, is find these consequent conjunctions." Hume argues that all of our reasoning is on the basis of arational responding.
▶︎ 33:36 Now, this is very important, it's arational. That's not irrational. Irrational means it's bad reasoning, you should not think it that way. Arational means it has nothing to do with reason. Your digestion is arational. When you go to the doctor and the doctor hits your knee with a little hammer and your leg goes, that's arational. You don't do that because you've thought, "I think the right thing to do here would be to move my leg, so I'm gonna do that as soon as he hits me." No, you just do it. Your leg does it.
▶︎ 34:16 That's your belief that the second billiard ball will move. You expect the second billiard ball to move the same way your foot jumps when the hammer hits it. It's a mental reflex. Not an argument, not an inference, nothing rational. Because there cannot be a reason for it, according to Hume. You cannot prove it, you cannot know it. There is no reason whatsoever to believe that the way things have been going so far are going to continue. But we do, just like my stomach digests food, just like my foot jumps when I get hit by the hammer, because that's the way my brain is constructed.
▶︎ 35:14 And that's fantastic. If we did not have that. See, this is what's so fascinating about Hume. Descartes, like many, many philosophers, says we should only believe what we can rationally prove. Hume says if you do that, you're gonna die tomorrow, because you're gonna be eating glass, you're not gonna be getting out of the way of cars, because you cannot rationally prove that those are the right things to do. That you should not be eating glass, that you shouldn't be getting out of the way of cars. If you succeeded in being perfectly logical, the human species would end immediately.
▶︎ 35:58 It is our lack of reason that is our saving grace. It is our arationality that enables us to survive in this world, not our rationality. Or he's not saying our rationality is bad. Of course it's very, very good, it's very helpful. But it only occurs on the basis of a foundation, here's a Cartesian idea, that is arational.
▶︎ 36:26 Descartes said the foundation has to be the most rational thing there is. Hume says the foundation has to be non-rational, or else we would not know how to get by. So the implications for this are that, first of all, what he calls his skeptical argument, that we do not have a reason to expect causal events to continue the way they have been, won't change our behavior. He says nature's too strong for reasoning. And that's great. It's really, really good that it is, or else we would perish immediately.
▶︎ 37:13 But the fact that we cannot even prove that we shouldn't eat glass tomorrow should give us a little humility about proving the existence of God, should give us a little restraint in our metaphysical yearnings for the ultimate truths of the universe. Let's pull back. Let's reconcile ourselves with our animal nature, and let's understand the way humans understand. We need to limit ourselves to the kinds of knowledge that we can have. What are those? Relations of ideas. Yep, we got those. Matters of fact with experience, and that's it. Those are the only kinds of knowledge that are legitimate for a human being, and metaphysics isn't like that.
▶︎ 38:16 When Descartes proved the existence of God a priori, what he was doing was, in the confines of his mind, he was making an argument, making a claim about something that existed in the universe. That's a contingent fact. There's no way he can know what's in the universe without empirical verification. And the idea that just by thinking he can plug in to the basic fabric of reality is hubris, is overweening pride and a waste of a mind that could have been used for much more useful things, according to Hume.
▶︎ 39:03 Now that we know what the mind, what is in the mind and what the mind, how the mind does reason, how it can reason, now we need to restrain ourselves only to those types of knowledge. And then, we will have a far more productive creation of knowledge and better science.