Michael: You probably think science is built on cold, hard proof. But what if its entire foundation—the idea that one thing causes another—is just a feeling? A psychological habit we can't shake, with no more logical proof than a hunch. Kevin: A feeling? Like, a hunch? That can't be right. The whole modern world is built on cause and effect. You flip a switch, the light comes on. That’s not a feeling, that’s a fact. Michael: Is it? Or is it just an expectation born from habit? That's the bombshell David Hume drops in his book, An Enquiry Concerning Human Understanding. Kevin: Hume... he's one of the big ones, right? The Scottish philosopher from the Enlightenment? Michael: Exactly. And what's fascinating is that this book, published in 1748, was his second attempt. His first, the Treatise of Human Nature, was a commercial flop—he said it 'fell dead-born from the press.' So he rewrote it to be sharper, wittier, and more explosive. This is the book that famously woke Immanuel Kant from his 'dogmatic slumbers.' Kevin: So this is the refined, weaponized version of his philosophy. He learned from his mistakes. Michael: He did. And he starts by building a very specific toolkit for thinking, a way to dissect our own minds before he turns that tool on the world.

Kevin: Okay, so what’s in this toolkit? If he’s going to convince me that causation is a myth, he needs some serious equipment. Michael: He starts with something that philosophers now call "Hume's Fork." He argues there are only two types of knowledge, two categories for every true statement we could possibly make. On one side, you have Relations of Ideas. Kevin: That sounds abstract. What does it mean? Michael: Think of math or logic. "2 + 2 = 4" or "all bachelors are unmarried men." These are things that are true by definition. You don't need to go out into the world to check if they're true. The opposite is inconceivable. A married bachelor is a logical contradiction. Kevin: Right, it’s self-contained. I can figure that out just by thinking about the words. What’s the other side of the fork? Michael: That's where it gets interesting. The other side is Matters of Fact. These are claims about the world. "The sun is shining," "this coffee is hot," "that bird is blue." To know if these are true, you have to rely on experience—on your senses. And crucially, the opposite is always conceivable. I can easily imagine that the sun is not shining. There's no logical contradiction there. Kevin: Okay, so one is about definitions and logic, the other is about what we observe in the world. That seems straightforward enough. Michael: It is. But then he adds his second tool, and this is the one that does all the damage. It's called the Copy Principle. Hume argues that every single idea in your mind, no matter how complex, is ultimately just a copy of a direct experience. He calls the direct experience an "impression" and the memory or thought of it an "idea." Kevin: So an impression is like seeing the color red for the first time, and the idea is remembering what red looks like later? Michael: Precisely. The impression is vivid, forceful. The idea is a fainter, faded copy. And his big claim is that you can't have an idea without first having an impression. It’s like he’s building a mental microscope. If you have a complex, fuzzy idea—like 'justice' or 'God' or, most importantly, 'causation'—Hume says, "Put it under the microscope. Trace it back. Where is the original, direct impression?" If you can't find one, the idea is meaningless noise. Kevin: Hold on, that seems too simple. What about something like a unicorn or a golden mountain? I've never had an impression of a golden mountain, but I can definitely imagine one. Michael: Ah, but you've had an impression of gold, and you've had an impression of a mountain. Hume says the mind's creative power is just to compound, transpose, and combine these basic materials that experience gives us. You're just mixing and matching your existing impressions. Kevin: Okay, I see. It's like a mental Lego set. I can only build with the bricks I've been given by my senses. I can't invent a new color of brick out of thin air. Michael: Exactly. And this simple, almost obvious-sounding toolkit—the Fork and the Copy Principle—is what he uses to dismantle our entire understanding of reality. Kevin: Alright, I'm intrigued and a little nervous. So what happens when Hume puts the big one, 'causation,' under this microscope? Where does he find the impression for that?

Michael: That's the million-dollar question. And his answer is what makes him so revolutionary. When Hume looks for the impression of causation, he can't find it. Kevin: What? How can you not see causation? I see it a hundred times a day. I push a key on my keyboard, and a letter appears. That's cause and effect. Michael: Hume would say you're not seeing what you think you're seeing. Let's use his classic example: two billiard balls on a table. You see the first ball rolling towards the second. That's one event. You see the first ball make contact with the second. That's a second event. Then you see the second ball roll away. That's a third event. Kevin: Right. The first one caused the third one. Michael: But where did you see the cause? You saw a sequence of events. You saw them happen one after another, in that order. But you never saw the "power," the "force," the "necessary connection" that supposedly links the first event to the third. You can't point to the moment and say, "There! That's the causation!" It's invisible. Kevin: Okay, my brain feels a little fuzzy. So if we don't get the idea of causation from a direct impression, where on earth does it come from? Because we all have a very strong idea of it. Michael: This is Hume's brilliant, and unsettling, conclusion. It comes from Custom or Habit. You've seen one billiard ball hit another a thousand times, and every single time, the second ball moves. Your mind, through sheer repetition, builds a powerful expectation. When you see the first ball rolling, your mind instinctively leaps ahead and anticipates the effect. That feeling of anticipation, that mental leap—that, Hume says, is the impression you're looking for. The "necessary connection" isn't out there in the world between the objects; it's a feeling of expectation inside your own head. Kevin: Wait a minute. This is huge. Are you saying that my belief that the sun will rise tomorrow has no more logical proof than my belief that my favorite sports team will inevitably disappoint me again? It's just... a habit based on past performance? Michael: According to Hume, yes. It's an incredibly useful habit, of course. He calls custom "the great guide of human life." Without it, we'd be totally lost, unable to function. But it's not based on reason. There is no logical proof that the future must resemble the past. We just assume it will because it always has. It's a psychological instinct, not a rational deduction. Kevin: This feels like it pulls the rug out from under the entire scientific method. Science is all about finding causal laws, predicting the future based on experiments. Is Hume saying that's all just a well-honed superstition? Michael: In a way, yes. A very, very useful superstition. He's not saying we should stop doing science. He's saying we should be humble about what it is we're actually doing. We're finding patterns and developing strong habits of expectation, not uncovering the secret, logical machinery of the universe. Kevin: Wow. So he builds this simple toolkit, and with it, he basically saws off the branch that all of Western science and philosophy is sitting on. Michael: And he was more than happy to pull that rug. Once he established that our belief in cause and effect is just a habit, he turned his new weapon on some of the biggest targets of his day: miracles and metaphysics.

Kevin: Okay, so if he’s already dismantled something as basic as one ball hitting another, I can only imagine what he does to the idea of miracles. Michael: His argument is devastatingly simple and flows directly from what we've just discussed. A law of nature, like gravity, is something we believe in because we have "a firm and unalterable experience" of it. We've seen it happen consistently, without exception, millions of times. A miracle, by definition, is a violation of a law of nature. Kevin: So it's a one-off event that breaks the pattern. Michael: Exactly. So Hume asks, what is more probable? That the entire, uniform experience of every human who has ever lived has been suspended in this one instance? Or that the person telling you the story is either mistaken or lying? Kevin: When you put it like that, the testimony of one person seems pretty weak against the testimony of, well, all of reality up to that point. Michael: That's the core of it. He says no testimony is sufficient to establish a miracle unless the testimony is of such a kind that its falsehood would be more miraculous than the fact it's trying to establish. It's an impossibly high bar, and he argues it's never been met, especially in the context of religion, where passions and biases run high. Kevin: He's applying his mental microscope again. He's weighing evidence based on experience. It's cold, but it's consistent. Michael: And he doesn't stop there. He takes this principle to its ultimate conclusion in one of the most famous, or infamous, paragraphs in all of philosophy. He essentially argues that if an idea can't be traced back to an impression (a matter of fact) or isn't a logical truth (a relation of ideas), it's worthless. Kevin: What does he say? Michael: He says, and I'm quoting here: "When we run over libraries, persuaded of these principles, what havoc must we make? If we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: for it can contain nothing but sophistry and illusion." Kevin: Wow. That's not just philosophy, that's a mic drop. He's basically saying most of the library is garbage. That's so aggressive and confident. Michael: It's an intellectual bonfire. He's clearing out centuries of what he sees as meaningless speculation. But it's important to understand he's not a complete nihilist. He doesn't think we should sit in a dark room, paralyzed by doubt. He advocates for what he calls "mitigated skepticism." Kevin: Mitigated skepticism? What's that? A diet version of despair? Michael: More like a healthy dose of humility. It means we should recognize the limits of our own reason. We should be cautious, proportion our belief to the evidence, and confine our inquiries to subjects we can actually investigate through experience. It's about being intellectually modest.

Kevin: So after all this intellectual demolition, after he's burned the libraries and dismantled causation, what's the big takeaway? Are we supposed to just live in constant doubt about whether the sun will come up? Michael: I don't think Hume's point is to paralyze us. It's to humble us. He reveals that the bedrock of our reasoning, the very thing that makes us feel so certain about the world, isn't some perfect, cold logic. It's psychology. It's habit. Our certainty is, at its core, a feeling of expectation. Kevin: And that’s supposed to be a good thing? It feels terrifying. Michael: But it can also be incredibly freeing. If you recognize that your own certainty is a product of habit, it protects you from dogmatism. It forces you to be more open-minded. You can't be absolutely certain, so you have to be curious. You have to keep testing your beliefs against experience. It's the foundation of a truly scientific and open-minded attitude. Kevin: So the destruction is actually a form of construction. By tearing down our false sense of certainty, he builds a foundation for a more honest, more humble way of knowing. Michael: Exactly. He’s not telling us what to think, but he’s giving us a powerful lesson in how to think—with skepticism, with humility, and with a deep respect for the limits of our own minds. It's a philosophy that challenges us to be less certain and more curious. Kevin: That’s a powerful thought to end on. It really makes you look at your own beliefs differently. Michael: It forces us to ask: how many of our own 'certain' beliefs are just strong habits we've never bothered to question? Kevin: This is Aibrary, signing off.