▶︎ 0:00 We're onto our second philosopher now, David Hume. He's an 18th century Scottish philosopher, and there was a survey given a few years ago to professors of philosophy, asking them which non-living philosopher they most identified with, they most felt captured their voice and their ideas, they're closest with, and Hume was number one. That's his popularity. Kant, who will be our next thinker, is number two. Nietzsche was 17. Our last one, Descartes, poor Descartes was only number 18, but that's still not bad.
▶︎ 0:40 We're going to be talking primarily about this book, the Inquiry Concerning Human Understanding, published in 1748, and there's good news and bad news about the book. The good news is there's really only one argument in it. So if you get that argument, you got it. You just have to understand that one argument. The bad news is, it's a big, complicated, multi-part, weird counterintuitive argument that takes a whole book to explain.
▶︎ 1:15 The first part of the book is preparing for the argument, setting up the pieces, defining the terms, explaining things, and in the middle is the argument itself, and then the rest of it is exploring the ramifications and the consequences of it. So that's going to be the way that we're going to be approaching it, is we're gonna do three phases: the preparation, which is boring, the argument, which is weird, and the conclusions, which are pretty neat. Unfortunately, you have to go through the first two to get to the third. But it has the capacity, and it has had this effect on many, many generations of people to fundamentally change the way you think about yourself and about human nature.
▶︎ 2:04 The book is titled An Inquiry Concerning Human Understanding. It's about understanding, therefore, again, it's about epistemology, just like Descartes was. But notice the qualifying word. This is an inquiry concerning human understanding. He wanted to put that in the title. That seems gratuitous. Why do we need that? What other understanding could there be? Why do we need to emphasize that this is human understanding?
▶︎ 2:34 Well, we do need to emphasize it. If you go back to Descartes, Descartes was actually not interested in human understanding. For Descartes, he pared us down to our essence, which is a thinking thing. Our humanity, our embodiedness, our living in the culture, in a society, in a history, none of that was relevant to thinking, for Descartes. In fact, really it was distractions. Being in a body, as many philosophers since Plato have felt, is just interference. It gets in the way with our thoughts.
▶︎ 3:24 And so for Descartes, if he were to write a title like this, it'd be an inquiry concerning understanding. An inquiry concerning the understanding of a thinking thing, whether that's a human or a computer or an angel or a porpoise, it doesn't matter. We want to just get down to the essence.
▶︎ 3:46 But Hume says, at the very beginning of the book, he says, this is on page four of the Hackett edition, "Be a philosopher, but amidst all your philosophy, be still a man." Hume thinks it's crucial that we maintain our humanity in our philosophy. The notion that we can get away from that, that we can cut that away and have anything left that's real, that's significant, that's important, that's truly us, is one of the professional deformations of philosophy, Hume thinks.
▶︎ 4:30 The only kind of understanding that we can understand, that we can use, that is important for us to think about, is our understanding. We have no idea what angelic or divine understanding is. We have no idea what an alien understanding is. The only understanding we know is human understanding, and for Hume, this has a significant effect on what thinking itself can be.
▶︎ 5:05 If we think about vision, how can we determine, if we had an inquiry concerning vision, or visibility, well, that's too vague, because a human vision is fundamentally different from an owl's vision or a bat's vision or a bee's vision. Each species, their anatomy, the way their eyeballs are structured, the way their brain is structured, determines what they can see, how they see it, how they decode it. The visual world that they live in is fundamentally different, as determined by their species.
▶︎ 5:49 Humans can only see a range of about 380 to 720 nanometers. That is the visible spectrum. And if it's lower than 380 nanometers or higher than 720, ultraviolet or infrared, it's invisible to us. We simply cannot see it. No matter what we think, no matter whether we think we should or we try, it doesn't matter, we cannot see it.
▶︎ 6:15 Hume thinks that the brain is an organ. It's a biological object in our skulls that also has capabilities and limitations the way our eyeballs do. Just like there are things that we can see and things that we cannot, well, there are things that we can think and there are things that we simply can't. There are things that we can understand because of the way that our brains are structured, and there are things that are beyond our ken. No matter how hard we try, no matter how hopeful we are, no matter what we think, we cannot think everything.
▶︎ 7:00 The problem is, if you are working on science, let's say, and you make a fundamental error, the world will correct you. If you build a bridge based on the wrong principles of physics, it'll fall down. You will get feedback. You will find out about your mistakes. But if you do metaphysics, if you speculate about the nature of divinity or the fundamental structure of the universe, you never get corrected. Nothing will tell you that you're wrong.
▶︎ 7:38 And what happens is, for all of history, there have been geniuses, brilliant people who could have benefited humanity in many, many ways, who spent their lives pouring their thoughts down the drain of metaphysics, wasting their time thinking that they could think thoughts they can't think. Like straining your eyes to see infrared. They thought they were capable of achieving metaphysical thought when, according to Hume, it's fundamentally inaccessible to human beings.
▶︎ 8:19 But how can we prove this? How can we know? If we don't get the feedback the way we get from the bridge falling down, then how could we find out? Metaphysics doesn't build anything, so nothing will break. That's why we've had so many people who have wasted all of this brain power on this futile chasing after divinity, after metaphysical truth that is simply not within our grasp.
▶︎ 8:49 Well, he has an answer. We are going to inquire concerning human understanding in the same way that we would find out the visible spectrum. To find that out, what would we do? We would study eyeballs. We would cut them open. We'd look at their structure. We would examine the visible portion of the brain. This is what we're going to do for the human understanding.
▶︎ 9:15 We're going to discern the anatomy of the mind. Not the brain. He couldn't do that. That was impossible at this time. But he could limn the details of the mind. He does what he calls a mental geography, a map of the mind. He's going to find the contents of the mind and the structure of the mind. What is in there and what we do with what's in there. And if we can figure that out, then we can know how far we can take that.
▶︎ 9:59 And what this will do, is this will draw a line. This will draw a limit. And a limit does two things. A limit tells us both what is inside and what is outside. It tells us both what we can do and what we cannot.
▶︎ 10:24 And if we can find this limit once and for all, then we can concentrate all of our effort as a species, all of the brain power that gets manufactured solely on what's within the limits, solely on what we actually can answer, and we can simply ignore what's outside that circle. This is what Hume wants to achieve with this book. This is what Kant will call critical philosophy.
▶︎ 11:04 Kant wrote three books, each one he called a critique. And Kant, as we'll see in the next lecture, was profoundly influenced by Hume, had enormous respect for Hume. He thought Hume didn't get it quite right, but he had the right problem. He had the right question, even if he didn't have the right answer.
▶︎ 11:20 And Hume didn't have a name for this, so we're going to use Kant's name, critical philosophy. What critical philosophy is, is understanding the structure and the nature of the mind so that we can draw its limits, and the limits tell us both what we can understand, what we can know, and what we cannot, so that we can give up the latter and focus entirely on the former. This is what his inquiry, Hume's inquiry is meant to produce for us.
▶︎ 11:53 Now, he's going to start this examination of the mind by looking at the contents of the mind. What's in our skulls? What's inside our head? What's rattling around in there? And like Descartes, like many early modern philosophers, the period from 1600 to 1800, he used the word idea. We have all these ideas in our head. But he says there's, of all the things in our head, there's one huge division. We can divide all of them into one of two categories, a kind of dualism.
▶︎ 12:32 We're not going to get into all the nitty-gritty, all the species. There were two major types. One is what he calls impressions, and an impression is when you're having an immediate experience of something. I am having an immediate impression of this pen. Try to pick up something and touch it, and that's an immediate impression.
▶︎ 12:59 Now, I put the pen down, and I recollect feeling that pen in the past when I was so much younger. It was a wonderful experience. And now I have an idea of the pen. I have an idea of the way it felt, and I have an idea of the way it looked. That is not an immediate experience of it. It's a memory. I can also anticipate it. I can't wait to touch that pen again. It's going to be so great. I'm imagining it. That's also an idea.
▶︎ 13:31 And if I go back and I touch it again, now I have an impression of it. So I'm having an impression of the pen. I put it away. I have an idea of it. These are both of the same thing. They have the same content, the idea of the pen, the impression of the pen, and yet they're fundamentally different.
▶︎ 13:49 The difference between ideas and impression for Hume is what he calls the relative force and vivacity. He's a wonderful writer. It's a little difficult for us, because it's kind of flowery, and it's kind of ornate. But he is a terrific writer. He was actually one of the first people to make a living off his own writing. This was a new thing. It used to be that you had to have a somewhat rich guy who would pay for you to, if you wanted to sit around and write all day.
▶︎ 14:27 Descartes wrote a horribly obsequious letter at the beginning of the Meditations, begging some rich people to pay for him to stay in bed until noon, thinking. And eventually, Queen Christina did it, and it didn't end well. But Hume wrote a multi-volume history of England that was a bestseller for years and decades and decades. And he lived off the proceeds of that. He's a terrific writer, but it's rather dated by now. And it can be a little bit difficult.
▶︎ 14:59 But he says the difference between ideas and impression is force and vivacity. So my impression of the pen and my idea of the pen, when I recollect that, are the same content, but the impression is much more vivid. It's, I feel it. It's right there. Think about when you're really, really hungry. And you are hungry, and the hunger is commanding your attention, and it is right there. There's no denying it. There's no getting away from it.
▶︎ 15:27 And then when you're not hungry, but you remember being hungry, you think about being hungry, and you have the same idea. You have the same content, but it has no grip on you. It's not pushing you around. It's not forcing you. It's weak. It's like a weak copy of something, like a faded Xerox of something. This is the difference between these two types of things that are in our head.
▶︎ 15:55 And Hume now takes this division, and he makes a claim. It's a very, very important claim. The claim he makes is that every idea you have in your head came from an impression. There is nothing, no idea in your mind that did not come from an impression. This is considered to be the founding theory of what's known as empiricism.
▶︎ 16:47 As you can see from the title, empiricism is the school of philosophy that thinks that empirical data is the key to knowledge. This is to be contrasted with rationalism, which, again, you can see from the title, focused on reason. Now, Descartes was the founder of early modern rationalism. Why? Because he said that thinking, reasoning, a priori knowledge was much, much more important than experience.
▶︎ 17:20 Remember? He said, "Yes, we need experience. We need sensory data in order to know the world, but we've got to translate that into numbers, so the mind then can grab onto it, so that we can reason about it instead of depending on the senses." Reason over sensory data for Descartes. Hume is saying the opposite. Hume is saying that the senses are far, far, far more important than reason.
▶︎ 17:51 And this debate between rationalism and empiricism is one of the defining debates of this early modern period, 1600 to 1800. Hume wasn't the founder of empiricism. Locke is often credited with that. But he's kind of the guy who took it to its conclusion.
▶︎ 18:14 You can see echoes of this, or anticipations of this all the way back to Plato and Aristotle, where Plato was a proto-rationalist and Aristotle would be a proto-empiricist. It doesn't map on perfectly, but this debate has been going on for a long time, but it really took center stage in the early modern period.
▶︎ 18:37 Hume is emphasizing that experience is a source of everything. He's inspired by John Locke, and John Locke had a very famous phrase where he said, "The human mind is a tabula rasa." You might have heard this phrase before. A blank slate. The human mind starts off, he says, as a blank piece of paper and experience writes on it.
▶︎ 19:18 And if there were never an experience, if you took a baby. This is just a thought experiment, it's never been done. If you took a baby and put it in a sensory deprivation tank right away, and kept it intravenously fed but never gave it any empirical experience, it would never have a thought in its head. You need experience, you need input from the senses to have any content in your head. Without that, there's nothing. You would remain a blank slate your whole life.
▶︎ 19:51 And Hume gives a couple arguments. Remember, for philosophy, you can make all kinds of statements, all kinds of claims, but you've got to have an argument for it. And he gives us two arguments. The first argument is, well, look, if you talk about direct experiences, if you have not had the impression, you will not have the idea.
▶︎ 20:19 And his example is pineapples. Pineapples were this new thing that they had. Some explorer had found pineapples and brought them back to England, and people were eating them for the first time. And before you tasted it, you did not have the idea of what a pineapple tasted like. The taste of pineapple simply was not in your head.
▶︎ 20:44 So you could look at it, you could examine it, you could put it under a microscope, you could get the chemical composition of it, you could learn anything about it. You could not know the taste of a pineapple until you put a piece in your mouth and got the actual impression. The second argument is more complicated.
▶︎ 21:08 He says, because an easy response is, "Well, hold on, Hume, I've got all kinds of ideas in my head that I've never had an impression of. I've got the idea of a unicorn. I've never seen one." And here's where Hume starts moving into the second aspect of the mental geography. The first aspect is the things in our heads, the ideas. The second one is what we can do with them, the powers or the faculties of the mind. How we affect them.
▶︎ 21:41 And he says the human mind, once we get an idea inside our heads, we have some power over it. We can take these ideas and play with them. So you can, for instance, combine ideas. If you've seen a horse, you've got the idea of a horse. You've seen a horn, you've got the idea of horn. Now your mind can glue those together and you've got an idea in your mind of a horse with a horn, and now you've got the idea of a unicorn. You didn't have a direct impression of a unicorn, but you had a direct impression of a horse and a direct impression of a horn, and your mind combined them together. So it's true that you didn't have an impression of the specific thing, but you had to have an impression of the parts of it.
▶︎ 22:32 The mind can do a lot of things with the ideas. You can make them big. You can take that idea of a unicorn, do it with me, and make it 100 feet tall in your mind. Now you've got a 100-foot-tall unicorn in your mind. All you did was take the thing and stretch it, like Silly Putty. You can make it purple. You can make it tiny. You can make it talk. You can do all kinds of things, but only because you have the idea of tall through an impression. You have the idea of purple through an impression. If you'd never seen purple, you couldn't have made that unicorn purple. But while you have it, then you take the unicorn, you take purple, you put them together.
▶︎ 23:14 So all kinds of highly abstract ideas which we think, which you could try to use as a counter-example, he will say are just constructions out of smaller pieces, where we had to have gotten the pieces from impressions. Once we have the pieces, then we can do all kinds of things with them inside the playpen of our minds. But everything has to be traced back ultimately to impressions. And this is one of the fundamental tenets of empiricism. Whereas Descartes thought, no, we have a priori ideas, we have these already in our heads, such as math. Now, what does Hume have to say about math?
▶︎ 24:05 This gets us to our second, the second part of his mental geography, which is continuing this idea of what we do with our ideas. His question now is, he's got his explanation of what is in our heads and where it came from. We've got ideas in our head and they all came from the senses. They had to. Now the question is, how do we think with those? How do we reason? What is the nature of inference?
▶︎ 24:47 And this is the second big piece of preparation before we get to the big argument. The first one is ideas and impressions and their relationship. Impressions have greater force and vivacity, and all ideas are derived from impressions. The second big piece of preparation, and the last one, is the kinds of reasoning that we have.
▶︎ 25:10 Now, of course, Hume accepts math. Math is very true. Math, he even accepts what Descartes says about math. It's absolutely certain. But there's a reason for that certitude. It's a very different reason than Descartes' reason.
▶︎ 25:32 So let's go back to our clear and distinct perception that we had of two and two equaling four. We know this with certitude. We know this for a fact. This is a necessary truth for Hume as much as it is for Descartes. But for a different reason. If we understand what this means, what this means is two and two is four, and four equals four. This is necessary because any other possible answer would be a contradiction. If you were to say, then you would get five equals four, and that's a contradiction because five doesn't equal four.
▶︎ 26:26 If you were to say it's three, eight, the color blue, anything else, any other answer isn't just that it happens to be wrong, like who the president happens to be or what color shirt I happen to be wearing, it's a contradiction. To say that anything equals four except four, you're committing a logical mistake. And we know that a priori that can't be right. You don't have to go check the world. You don't have to do a bunch of experiments of adding things together to see if two and two equals four because nothing else could equal four except four. One and only one answer is non-contradictory. And therefore, we can know these a priori. Descartes was right to that degree.
▶︎ 27:23 But the reason why we know this is because it's empty, is because it says nothing. What does it say? It says that four is four. Yeah. Okay, it's true. But that tells me nothing about the world. It doesn't tell me if there are four anything. It doesn't tell me if there's two anything and two anything or four any of. It just tells me that this idea, four, is the same as itself.
▶︎ 27:54 For Hume, these, he calls these relations of ideas. Because all we're doing is we're taking ideas and putting them into relationships. Relationships of identity. Relations of ideas are absolutely certain, we'll give Descartes that, but that certainty is purchased at the price of saying nothing. It's purchased at the price of being completely uninformative. I know it's true, but I know nothing because of it. It gives me no knowledge. It's a pyrrhic epistemological victory, according to Hume.
▶︎ 28:51 The other kind of knowledge, what he calls matters of fact, these are completely different. Now matter of fact is informative. It tells us something about the world. But the only way that something can actually be informative is if more than one answer is possible. If only one answer is possible, then it doesn't tell you anything to tell you that that answer's true because nothing else could be. It's informative if it rules out other things that are actually possible.
▶︎ 29:40 So my shirt is blue. This is true. The reason why it tells you information is because it could have been red, it could have been polka dot, it could have been white, it could have been a bunch of other colors. All of those other colors are possible. They're not true. They're not actual. They didn't happen, but they could've. Every one of them is consistent, logically consistent, coherent, unlike three equaling four. Three equaling four is impossible. It's not just that it happens not to be the case. It's impossible. My wearing a white shirt, it is entirely possible it didn't happen, but there's no problem with it. It's completely intelligible, coherent, could easily have happened.
▶︎ 30:33 And it has to be this way. This is what's so interesting here. These relationships of these properties of these two different types of information are internal. Relations of ideas only get certainty, a priori certainty, because they're uninformative because it couldn't be anything else. That's the reason why you know it without looking at the world. It's a priori. And that's the reason you know it for sure because nothing else could possibly be true.
▶︎ 31:08 Matters of fact are only informative because other things could be true. But that also means other things could be true. So you have to find out the fact. You have to check the world, which means they're empirical. Relations of ideas are a priori. I don't have to look to see if four equals four. I don't have to take a survey and make experiments. It couldn't be anything else. I can know that in the safety and comfort of my mind. But I can't know what shirt you're wearing without going and looking at it, because you could be wearing a bunch of different types of shirts. If you have four shirts, I know you have four shirts without going and looking at them. But what color they are, I cannot know that without seeing.
▶︎ 32:02 These relations of ideas are absolutely necessary. That's why I can know them a priori, and that's why they tell me nothing. Matters of fact are empirical because there's something else, and it's a very important term. They are contingent. To be contingent means could have been a different way. It isn't. I did wear a blue shirt, but I could have worn lots of others. So contingency and the need for empirical verification are necessarily intertwined. Because you could
▶︎ 32:57 be wearing a whole bunch of different color shirts, I cannot just think in my mind and logically infer which color it is because it could be blue, it could be black, it could be white, it could be all these different things. Because this is absolutely necessary, I can infer in my mind that if you have four shirts, you have four shirts. So these are a priori, and these are empirical because these are contingent.
▶︎ 33:28 Now, for Descartes, the a priori beliefs, the mathematical beliefs, these were the crucial ones because they had that absolute certainty. They resisted that truth, that doubt. That was what he was after. Hume says you paid an awfully high price for that. The things that matter, the things that we care about, the things that tell us what the world is like, the things that guide your actions are all matters of fact.
▶︎ 33:58 Science is all matters of fact. Math is just a way of putting or just relating them. And just everyday common beliefs, the actions, the beliefs you act on are all matters of fact. What are you going to eat? What is safe? Who are you gonna see? Where do you live? All of these are matters of fact.
▶︎ 34:23 So matters of fact are vastly, vastly, they vastly outnumber relations of ideas, and they're vastly more important. They're also weirder. There's no mystery as to how we know relations of ideas. Couldn't be anything else. It's guided by the law of non-contradiction, one of the most fundamental logical laws there is, the law of identity. It's four equals four. It's identity. So we know how we know those.
▶︎ 34:55 But the question that Hume is asking, the inquiry is how do we know matters of fact? These are the ones that are important. These are the ones that make up science. These are the things that guide our every action. So how do we know them? What is the reasoning that underlies them? And his answer is very, very surprising.