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[intro music] My name is Jennifer Nagel. I teach philosophy at the University of Toronto, and today, I want to talk to you about the problem of skepticism. What do you know for sure? Consider the fact that you are watching a video, on your computer, right now Is this something you "know?" Before you say yes, consider the following question: do you think it is possible for someone to dream that they're watching a video online, when in fact, they're asleep in bed, with their computer turned off. Can you prove that you are now awake and not dreaming? If not, do you know that you are watching a real video as opposed to dreaming one up? If you start feeling inclined to doubt that you have knowledge, you're feeling the attraction of skepticism. Our word, skepticism, comes from ancient Greece, the home of not one, but two great skeptical traditions, academic skepticism, and Pyrrhonian skepticism. Academic skeptics argued that sensory impressions, which are often taken to be the foundation of knowledge about the world, don't actually enable you to know anything. Do you have the impression that the voice you're hearing now, is the voice of the same person who narrated the first video in this series? I might have an identical twin. You might be mis-remembering, dreaming, or in some other way making a mistake. Because impressions can be misleading, you can't know that the same person is narrating both videos. The academics used arguments like that one to support their general conclusion that knowledge of the world is impossible for humans. Pyrrhonian skeptics went one step further. Their mission was just to keep on inquiring, and doubting everything without reaching any conclusions at all. Where the academic skeptics argued that knowledge was impossible, the Pyrronian skeptics worked to suspend judgment even on that point, keeping all questions open. Some skeptical arguements have been known since antiquity and used in both eastern, and western philosophy. Most famously, the dreaming argument. If what you are now experiencing is just a dream, then it's not clear that you know anything about your immediate environment Or even about yourself. The ancient Chinese philosopher Xiang Xiu [Jwang Ju], reported having dreamed that he was a butterfly, and worried later that he did not know whether he was then, a man dreaming he was a butterfly, or whether, he might now be a butterfly, dreaming he was a man. You might think that there are some facts you could know whether or not you are dreaming. The 17th century philosopher, Rene Descartes, suggested that even if you are dreaming, you should still be able to know that a square has four sides, or that 2+3=5. But Descartes found a way to raise skepticism about those facts, too. He noted that it feels natural to us to make those simple mathematical judgments, but pointed that we could ask whether what feels natural to us really has to be true. Where does our nature come from anyway? Decartes also developed a powerful skeptical scenario, designed to make you doubt absolutely everything, including your grasp of abstract fact. Imagine that there is an evil genius of utmost power and cunning, devoted to deceiving you. The evil genius controls all your sensory impressions and all your instincts, about math, and geometry, and so on. Making false things seem true to you. The challenge of skepticism, Descartes argues, is the challenge of proving that you are not, right now, in the hands of such a demon. In the next video, we'll look at Descartes' own way of answering that challenge. Meanwhile, various other powerful skeptical arguments have emerged since Descartes' time. The 18th century philosopher David Hume, had some especially good ones covered in detail in two separate Wi-Phi videos. Moving to the present day, we have a new version of Descartes' evil genius argument. Imagine a brain kept alive in a vat, and connected to a supercomputer that delivers sensory signals to simulate the experience of a coherent reality. The computer also picks up the brain's outgoing motor signals and adjusts its inputs accordingly. When the brain sends out motor signals to raise a hand and touch something, the computer delivers coordinated visual and tactile input of seeing the hand and feeling what it touches. If the computer program is good enough, and let's assume that it is, the brain-in-a-vat experiences a perfectly realistic virtual world. He could have experiences of going to the beach on a sunny day, meeting friends, being stuck in traffic, or being home alone, watching videos about philosophical topics. Is there anything you could point to, in your present experience, to prove that you aren't a brain in a vat? It won't help to pinch yourself. The local feeling of pain, is just the kind of sensory signal that the supercomputer can easily supply the in-vatted brain. Typical skeptics don't try to prove that you actually are a brain in a vat, they will argue instead that it's bad enough that you just might be, but you can't tell the difference. Even if you are in an ordinary physical world, watching a video, and actually looking at a hand in front of you -- we call that the good case -- it's a problem that your experience feels just like the experience of the brain-in-the-vat. He doesn't know that his hand is in front of him; he's just a brain. He doesn't even have hands. We call this the bad case. So, even if you are in the good case, and your experience really is coming from looking at your hand, You're just lucky that you're not in the bad case. You can't prove that you aren't, and your inability to rule out the bad case means that you don't actually know that your hand is really there in front of you. The dreaming argument the evil genius, and the brain-in-the-vat scenario are known as "global skeptical scenarios." They raise doubts about virtually everything you would ordinarily take yourself you know. But skepticism doesn't have to be global. You can raise skeptical worries about some particular range of knowledge. For example, you can worry about your knowledge of the past. What if the whole universe just came into being five minutes ago, complete with fossils, antique-looking furniture, and your own apparent memory traces? If it did, it would look just the way it does now. But many of your beliefs, like your beliefs about what you did last summer for example, would be false. In a more restricted, local skepticism, we can raise skeptical worries about knowledge of single facts, just by thinking of some possible way in which things might fail to be as they appear. Consider Alice, who's walking down the street and wondering what time it is. She glances up at the clock, and sees that the hands show 4:30. Suppose that's right, and that the clock is working fine. Ordinarily, we'd say, "Now Alice knows that it's 4:30." But if we highlight something that could have gone wrong -- sometimes clocks are broken -- and Alice didn't look long enough to be sure that the hands were moving, then it gets harder to see Alice as really knowing the time. If her quick glance isn't enough to tell a difference between a working clock and a broken one, then how does Alice really know what time it is? Just thinking about the possibility of error can make it seem like knowledge is really hard to attain. Do we always have to double check that the clock is working in order to know the time? It's surprisingly easy to generate doubts about human knowledge, even knowledge of the kinds of things we'd ordinarily consider easily known, like whether there's a hand in front of your face right now. Skepticism, whether it's global, or local is an ancient, and difficult problem. Philosophers have proposed various solutions to this problem. The next two videos describe some of the main ways of answering the skeptic's challenges and defending the idea that knowledge is humanly possible. translated by: Seohyun Yoon(윤서현)