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<The following is a portion of Larrys Text, wikification is encouraged>

In its broadest sense, skepticism is the view that either we do not have any knowledge, or that we cannot have any propositional knowledge -- knowledge either about everything, or about some particular area. We're going to have to narrow our investigation; but first let me explain the different kinds of skepticism.

First, skepticism can be either the claim that we don't have knowledge, or that we can't have knowledge. There?s a difference -- the second is a stronger claim, and harder to prove. I mean, it really is one thing to say that we could, but unfortunately don't, have knowledge. I think Socrates might have held that view. He basically seemed to think that if we continue to ask questions we might eventually come to have knowledge; but we don't have it yet. (Or they didn't back in ancient Greece anyway!)

It's quite a different thing to say that we couldn't ever possibly have knowledge -- to say that knowledge is impossible. I think this has been, believe it or not, a more common opinion among skeptics. They really did, and a very few still do, think that we just cannot know anything. This is the variety we'll be investigating in a little bit.

Now remember that I said that skepticism can be either about everything, or about some particular area. If knowledge of anything at all is impossible, then my view is global skepticism. Whatever in the world you pick, I will say that you can't possibly, or at least don't, know it. There have been very few global skeptics in the history of philosophy. Hardly anybody has been that bold. Global skepticism really is bold -- because look at what it denies. It denies I know my own name. It denies that I know that I have a mind, or a body. It denies that I know I have been alive for longer than ten minutes. And so forth! How anyone might rationally support global skepticism, I won't venture to say. I seriously doubt there are any even minimally plausible arguments for global skepticism, at least of the variety that says: "We cannot know anything at all." Maybe the weaker versions, that say, "We do not know anything at all" could have some support. But I'm not going to get into that.

Now if I say deny that we do or can have knowledge of a particular area, then my view is local skepticism. And I say that I am a skeptic about the area that I have doubts about. Of course there are different kinds of local skepticism, depending on the area. Areas like: the external world; other minds; the past and the future; and so forth. Take for example the external world. If I say that no one can know anything about the external world, the world that exists apart from my own mind, then I am a local skeptic, and I espouse skepticism about the external world. Or even more briefly, external world skepticism.

So let me summarize this dry introductory material about skepticism, and then we'll get into the meatier stuff. Skepticism is the view that either we do not have any knowledge, or that we cannot have any propositional knowledge -- knowledge either about everything, or about some particular area. We are going to talk about the sort of skepticism that claims we cannot have propositional knowledge. Skepticism about everything is global skepticism, but that's a very implausible view. We are instead going to talk about some different kinds of local skepticism. So we are going to look at some skepticisms that say: We cannot have knowledge about some particular area, X, or Y, or Z. (I?ll say what X, Y, and Z might be in a bit.)

Now remember, just before we started talking about skepticism, I said we would ask: "Is knowledge possible?" I said we could rephrase that by asking: "Am I ever sufficiently justified in believing something in order to have knowledge?" I said the skeptic says "No." So the skeptic says I am never sufficiently justified in believing something in order to have knowledge of it. But we just learned that this is a little vague, and we are narrowing down what the skeptic claims. So more precisely we'll say the skeptic claims:

We can never be justified in believing something about area X, or at least not enough to give us knowledge about that area.

You're probably wondering what "areas" I'm talking about, exactly. Once again the areas are, for example, the external world; other minds; the past and the future; and so forth.

Now remember some things we talked about last time, about justification. At one point we were talking about foundationalism, and I gave an argument for foundationalism. If foundationalism is true, then there have to be some basic beliefs; and basic beliefs, remember, are beliefs that are justified, but not justified by other beliefs. So what could justify basic beliefs? I said: mental events, like an instance of perception, or an instance of memory.

Let me put that point in a more concrete way. Say I'm looking across a field of daisies and I see a cow. So I believe there is a cow across the field of daisies. And surely this belief is justified. How is it justified? What justifies the belief? Why, the mere fact that I see the cow. Or more technically: it is the event of my seeing the cow that justifies my belief that the cow is there peacefully grazing on daisies.

This is an important point to understand so I'll give you another example. Suppose I am reminiscing about my high school days and I vividly recall a very nasty gym teacher -- very loud and rude. So I believe I had a nasty gym teacher; and again this belief is justified. How? By the fact that I remember it. That's all. Again, more technically: it is the event of my remembering my nasty gym teacher that justifies my belief that I had that teacher.

This is all actually very straightforward, once you understand what's being said. If we assume that foundationalism is true, then we?ve got basic beliefs; and our basic beliefs, to be basic, have to be justified by something that isn?t a belief; so what justifies them? The operation of ordinary cognitive processes, like seeing, remembering, feeling, introspecting, and so forth. When I remember something, that gives me excellent reason to believe what I remember. Not always of course, but usually, especially if the memory is vivid and I can't think of any reason to think that this particular memory is wrong.

But in any case, if I do get justified beliefs from the use of memory, then my memory has to be reliable. Similarly with perception: if my seeming to see something makes me justified in believing it's there, then I have to assume that perception is reliable. If it were unreliable -- if it were often giving me false information -- then I couldn't say I was justified just based on the use of perception.

You'll remember that I said that local skepticism is skepticism about particular areas. Now I can tell you what the areas are: they are matched up, not exactly but fairly closely, with different cognitive processes. What the skeptic doubts is that our cognitive processes are reliable. The skeptic says, for example: perception is not reliable; therefore, you are not justified in your beliefs about what you perceive.

Since what you perceive is the external world, this sort of skeptic says: you are not justified in your beliefs about the external world. So one kind of skepticism is called external world skepticism: that is the view that we cannot know anything about an external world, even that such an external world exists! The reason we can't is that our faculty of perception is not reliable. We have already seen this view in action, when we talked about the theory of perception. Now we're approaching it from a different angle.

You might be wondering, of course, why anyone would want to say that perception is not reliable. I've already given you one argument, which was due to David Hume. To review that very quickly, Hume's argument basically says that we can't know anything about the external world, because to know that we would have to know that there is a connection between our sense-data and the external world that they are supposed to represent. But the only thing we have contact with are our sense-data; we can never know anything in the external world except by first knowing our sense-data. But then we have no way to prove the connection between our sense-data and the external world. So we have no way to prove that our sense-data do represent any external world -- and that is to say that we have no way to prove that perception is reliable.

Now I'm going to give you another argument for external world skepticism, which is more famous than Hume's. Namely, Descartes' famous dreaming doubt, and it goes like this. Descartes was writing one evening in his room, and he thought to himself (I paraphrase very loosely): What if I am asleep in bed right now, and only dreaming that I am awake, and writing? Isn't that at least possible? Then he said, well surely, I can tell when I am awake and when I am asleep. I can tell the difference between wakefulness and a dream. All sorts of strange things happen in dreams; I pass unaccountably from scene to scene when I'm dreaming; I don't have any long memory of what happened in a day, when I'm dreaming; and so forth. Then Descartes said: Haven't I had those very thoughts in some of my dreams? Sometimes, when I was dreaming, I was convinced that I was awake! I even tried to test that I was awake, when I was dreaming, and the tests convinced me that I was awake! But I was wrong; I was dreaming. Isn?t it quite possible that the same thing is happening to me right now? Isn?t it possible that I am dreaming that I can test whether I'm awake or asleep -- and of course, in my dream, I pass the test? So it seems really vivid to me right now that I'm awake -- but in fact, I'm asleep?

Well, Descartes said to him, I guess there aren't any definite signs, or tests, that I could use to tell whether I'm asleep or dreaming. I could, after all, be dreaming those very tests. I have experience of doing that, thinking that I passed the test for being awake, when really I was only dreaming. So there isn't any way to tell that I am awake now. I cannot possibly prove that I am awake. So, Descartes said to himself, I don't really know that I am awake now and writing in the evening. For all I really know, I could be asleep. That's Descartes' dreaming doubt.

Now we can go on and give this argument some more detail. For one thing, why does Descartes think that he doesn't know he's awake and writing? Well, he might be asleep. But what difference does that make? The difference that it makes is that his faculty of sense-perception would not be reliable if he were asleep. In other words, if he were asleep, it would seem to him that he is seeing, feeling, and hearing various things; but he wouldn't really be. In that case, of course, his faculty of perception wouldn't be reliable. But Descartes appears to go farther than that: he appears to be saying that since he might be dreaming, since he can't rule out the hypothesis that he is dreaming right now, that also means that his faculty of perception is not reliable.

If you follow what Descartes is saying, you very well might find it exasperating, or silly. "I mean, of course I can tell that I'm not dreaming": that's what you want to say. Here is Descartes? reply: maybe you can, but maybe you're just dreaming that you can tell. If you say you can tell the difference between being awake and being asleep, then you are assuming that you're awake! And so you're begging the question against the skeptic!

Or maybe you?ll say this. I can tell that my sense-perception is reliable. Here's how I can show that it's reliable. When I see something, like that cow chewing on daisies, I can go over to the cow, touch it, hear it, lean on it, and so forth. That confirms that really am seeing the cow. In the same way, when I hear something, like a marching band outside, I can step outside, and look at the marching band, talk to the members of the band, and so forth. That confirms that I heard the band outside. Throughout my life I've had so many experiences like this that I am practically certain that, in the more obvious cases anyway, my faculty of perception works -- it's generally reliable.

Well, I'm sure you can guess how Descartes' skeptic will reply to this. You might just be dreaming that you are touching, hearing, and leaning on the cow. That marching band might just be part of a dream. For that matter you might only be dreaming that your faculty of perception has been generally reliable. If you argue: I'm not dreaming, because my faculty of perception is reliable, then you are begging the question! First, you have to establish that you're not dreaming! And that's impossible! So you can't know that your faculty of perception is reliable.

And a sharper skeptic might make another remark about seeing the cow and hearing the marching band. Because, after all, weren't you using sense-perception in order to try to argue that your faculty of perception is generally reliable? Think about that: in order to show that your sense of sight works, you use your sense of sight and other senses; in order to show that your sense of hearing works, you use your sense of hearing and other senses. And it's not like you can avoid that. It would be really bizarre (though some philosophers have actually tried it) to try to argue that your senses are reliable, without making use of your senses. But if you make use of your senses, aren't you begging the question again? Don't you have to assume, or presuppose, that your senses are generally shipshape before you start using them to prove anything, including whether your senses are generally shipshape?

Do you see the general problem? How can you prove that perception is reliable without using your senses? That seems impossible. But how can you use senses without assuming that perception is reliable? If you do that then you're arguing in a circle, you're begging the question. So what?s the upshot? That you can't prove that perception is reliable. If you try, you beg the question, and question-begging just isn't allowed.

Notice that this is actually a third skeptical argument, different from Hume's and Descartes', although it is related to both. Hume said you can't prove that your sense-data represent the external world; Descartes said that you can't even prove that you're not dreaming; and this third argument says that you can't prove that perception is reliable, without assuming that your senses are reliable and thereby begging the question at issue.

This third argument is also very serious because it can be used to generate skepticism about other of our cognitive processes. Such as memory. Do you think it would be possible to prove that your faculty of memory is reliable? Well, how would you do it? Could you even possibly do it without relying on any memories at all? Because if you do rely on any memories, then you're assuming that those memories are reliable: and that's what you're trying to prove, so you can't assume that. But how could you possibly show that your memories really do represent the past, just by the use of your other cognitive processes, such as perception, introspection, and so forth? Seems to me that you couldn't prove that. Not without begging the question.

Now let's see how we can answer skepticism. I am very confident that we can, and that skepticism does not really pose a threat to our knowledge-claims. Let me point out a few very important objections to skepticism, that show that it may not be nearly as serious as you might think.

First of all, in all three arguments -- Hume's, Descartes', and the circularity argument -- the claim is made that we can't prove something or other. We can't prove that sense-data represent an external reality. We can't prove that we're not dreaming. We can't prove that perception, or memory, is reliable. But now ask yourself: just because you can prove something, does that mean that you don't know it? Or that you aren't justified in believing it? Take Descartes? dreaming doubt as an example. Suppose you're convinced that you can't prove that you're not dreaming, not without begging the question. And you're even willing to admit that mere very slight possibility that you are dreaming right now. However, you might reply, who cares? So what if I can't prove, to Descartes? skeptic, that I'm not dreaming? Who cares if there is a very, very slight possibility that I'm dreaming right now? Does that really matter to my knowledge-claims?

Now, Descartes himself thought it definitely did matter. Descartes wanted absolutely certain knowledge -- knowledge beyond any doubt. And so he thought that if you can raise the smallest doubt about something, then you don't really know it. For example, the dreaming doubt raises the very small possibility that I am not standing up right now, talking to you; I might be dreaming; and so Descartes would say (at that point -- later he thought he refuted this skepticism) that I don't know I'm standing up right now.

So this forces us to ask ourselves: Do we have to have absolute certainty, lacking any doubt whatsoever, in order to have knowledge? That would be the absolutely strongest grade of justification possible. And then we would be saying that knowledge is not just sufficiently justified true belief, but certainly true belief.

Now I personally don't think that such a strong degree of justification is necessary for knowledge. After all, I think we can know what the weather is going to be like, just by reading the morning forecast! Sometimes we're wrong; but if we're right then we have knowledge. So I'm not particularly worried if I can't prove that I'm not dreaming. I think it's extremely unlikely that I'm dreaming, and I think I'm perfectly well justified in thinking I'm awake. And I don't have to know with absolute certainty that I'm awake, of course, to be well justified in believing I'm awake.

You should know this, that Descartes himself rejected his skeptical doubts in the end. But he thought he could prove that his life is not just a long dream. His procedure was first to prove that God exists, and then to say: well, God is not a deceiver, he is a good God. So he wouldn't allow the possibility that I'm asleep when by every indication I'm awake. And besides, he gave me a faculty of sense-perception, and certainly God wouldn't make this faculty so faulty that it is unreliable. So my faculty of sense-perception is reliable. So Descartes made God the guarantee of his being awake, and of the reliability of his cognitive processes.

Of course, a lot of people have disagreed with Descartes on these points. I won't get into their reasons why.

Here's a second thing you might observe about skepticism: the skeptic makes absolute certainty a requirement for knowledge, then you could reply. So apply that observation to skepticism itself. Is skepticism itself entirely beyond doubt? Isn't it possible to raise various kinds of objection to skepticism? So it would appear; but then no one can know that skepticism is true. So then the skeptic can't know that skepticism is true. But this is actually a bit of a weak reply, because it doesn?t really refute skepticism. The skeptic, after all, may be perfectly happy to admit that no one knows that skepticism is true. The skeptic might rest content saying that skepticism is very probably true. That's not the kind of claim that we non-skeptics will be happy to allow.

A third objection, which especially applies to the circularity argument, comes from that common-sense Scotsman, Thomas Reid. Reid argued as follows. Suppose the skeptic is right, and perception is not reliable. But perception is just another one of my cognitive processes; and if it is not reliable, then my others are also bound not to be reliable. All of my faculties came out of the same shop, he said; so if one is faulty the others are bound to be as well. But that means that the faculty of reasoning, which the skeptic uses, is also bound to be unreliable too. In other words, when we reason, we are bound to make errors, and so we can never trust the arguments we give for any claim. But then that applies to the skeptic's argument for skepticism! So if the skeptic is right, we should not pay attention to skepticism, since the skeptic arrives at the skeptical conclusion by reasoning. And if the skeptic is wrong, then of course we need not pay attention to skepticism. In either case, we need not take skepticism about the reliability of our faculties seriously.

Do you recognize the form of Reid's argument? It's a dilemma, like this: if P, then Q; if not-P, then Q; either P or not-P; therefore, in either case, Q. Either the skeptic is right, in which case we can't trust our ability our reason and so can't trust the skeptic's conclusion; or the skeptic is wrong, in which case again we can't trust the skeptic's conclusion. In either case we don't have to worry about skepticism!

Well, as you know we could go on nearly endlessly about this topic. It's very interesting and has received a lot of discussion in philosophy, so there are a lot of arguments on both sides. I've only given you a basic sampling.