 Man, it's going to take me an extra half hour to get to the airport. Apparently, the construction crews are debating whether knowledge is even possible, and so there's been some sort of epistemic closure. Last episode, I mentioned Bayesian epistemology, and I realized shortly after I posted it that I never really told you what that word means. Let's do a quick crash course about what it is, why it's the focus of so much thought in philosophy, and some interesting complications, because that's what makes philosophy fun. Epistemology is the body of thought about knowledge, scientific knowledge about which planet is closest to the sun, theistic knowledge about what gods do or don't exist, mathematical knowledge that three is a prime number, practical knowledge that Carol is bringing her famous casserole to Thanksgiving, moral knowledge that we should pretend to like it to avoid hurting her feelings. Most of us would claim some ability to know about those sorts of things. We use the same phrase to describe the same sort of relationship in each of those statements. I know that X. That relationship is, in many ways, absolutely essential to how we interact with and understand our world. As anyone who's been paying attention to current events in media and politics is no doubt aware, having the ability to reliably differentiate between knowledge and opinions or feelings or other mental phenomena is really, really important. To figure out what knowledge is, let's try to break it in various interesting ways and see what really makes it tick. First, take the statement, I know that this marble is white. That obviously doesn't work because it's wrong. The marble is black. So for something to count as knowledge, it would seem that it has to be true. No problem. Second, imagine that I'm trying to mislead you about the color of the marble and I say, I know that this marble is white. Well, unfortunately I've screwed up the magic trick and it actually is white. But I didn't know that, despite my statement being true. In order for it to count as knowledge, I have to believe that it's true. Third, imagine that I watch someone mix the marbles up and then conceal one. Then I say with conviction, I know that that marble is white. And I'm right. But when someone asks me how I knew that, I say, well, the invisible purple unicorn told me. You probably wouldn't say that I actually knew what color the marble was, seeing as I've lost my marbles entirely. My belief was true, but it was only true because I got lucky. There seems to be a necessary connection between the truth of a belief and the belief itself, some sort of valid, rational justification. So in order for someone to know something, it has to be justified. It has to be true. And the person in question has to actually believe it. Plato figured that much out about 2300 years ago, and while there have been various debates about the mechanisms of acquiring and processing knowledge, justified true belief has been more or less the whole story for a long time. But around the beginning of the 20th century, some philosophers discovered yet another way to break things, and they still haven't really agreed on how to fix them again. Let's say that you see I have a jar full of nothing but white marbles. I reach down and get a marble and ask you what color it is. And you understandably think that it's white. You say, I know that that marble is white. And sure enough, it is. But you didn't see that I actually have a jar full of black marbles down here too. Now, you believed that the marble was white, and that belief was true. And it definitely appeared that that belief was justified. After all, if I only had a jar of white marbles, it seems perfectly rational to say that any marble I'd take out of it would be white. But it seems like you were only really right by accident. After all, I could have taken a marble from the other jar. Did you really know what color this marble was? These sorts of problems, known as Gettier or Gettier problems, kind of throw a wrench into the works for traditional epistemology, as it seems that justification, truth, and belief aren't sufficient criteria on their own to count something as knowledge. There are numerous plausible sounding responses to Gettier problems. You could try adding a fourth criterion. You could try making the justification bit a little more robust, or replace it entirely with something else. Many brilliant philosophers have been grinding away at it for decades. Now, while they've had all sorts of great ideas and insights, nobody's really nailed down a definition for knowledge that matches our intuition in every conceivable case. That's fun, but let's do something a little more radical. We've talked a little about skepticism before, an attitude or process of doubting certain ideas. Skepticism is a natural part of epistemology because it brings knowledge into question, essentially asking, do you really know that? The most extreme version of skepticism is academic, or Cartesian skepticism, the often ridiculed philosophical position that all knowledge is impossible. The prototypical example of academic skepticism was written by René Descartes, where he rejected everything that had even the most remote possibility of being false, leaving him with only one certain fact. I think, therefore I am. Even that runs into some potential problems if you look closely enough. But the idea that we can only believe something if there are no conceivable grounds for doubt? That's a bit peculiar. It's certainly not how we usually treat knowledge, and given the unintuitive nature of academic skepticism to begin with, we might well be justified in rejecting the eliminate all doubt test. But there is a much subtler version of academic skepticism that depends on very simple rules of logic and a very straightforward epistemic idea called the closure principle. First, let's review our modus tolens. If A, then B, not B, therefore not A. If it's pizza, it's cheesy and delicious. It's not cheesy and delicious, therefore it's not pizza. Piece of cake, or pizza. Next, let's take a look at the closure principle. If someone knows something, P, and that entails some other bit of knowledge, Q, then they also know Q. If I know that I'm going to go to an Indian buffet for lunch and having Indian buffet means that I won't be hungry for dinner, then I also must know that I'm not going to want dinner. That's also pretty easy. I don't see what all the fuss is. Oh. Oh, dear. If I know that the marble is white, that entails that I also know that I'm not just a brain in a jar somewhere hooked up to a simulation of the real world, but I don't really know that I'm not just a brain in a jar. Therefore, I don't really know that the marble is white. You can use that same structure of argument for literally any bit of knowledge you can think of. I know that Mercury is the closest planet to the sun. I know that Carol is bringing casserole. It's equally applicable to all of them. And it's kind of hard to find a way that it doesn't work. I mean, you could try to find a problem with the closure principle, but it does kind of seem unimpeachable. And if you try to poke holes in the modus tollens, you're sort of dismantling the entire framework of logic and reason to begin with. So, yeah, maybe knowledge is impossible. I mean, very few philosophers are eager to bite the bullet on that one. And there are some very compelling responses to academic skepticism under the closure principle, but it's still an interesting problem to wrestle with. I've only really given you a glimpse of the surface of the iceberg that is epistemology, where knowledge comes from, how we get it, what's necessary for us to get it, what kinds of knowledge exist. There are all sorts of fascinating and incredibly nuanced questions revolving around the idea that people can sometimes know things, or maybe not. But what do you expect from a field of study as old as Socrates? After all, he claimed that he didn't know anything. But now that I think about it, he never really went about proving it. What do you think counts as knowledge? How would you respond to academic skepticism under the closure principle? Please leave a comment below and let me know what you think. Thank you very much for watching. Don't forget to blah, blah, subscribe, blah, share, and don't stop thunking.