OK, we've gone through all four parts of the definition of "epistemology" I've given you. I want to spend a few minutes on a topic that is closely related, and which sheds light on a lot of things we've talked about.
Here's another problem, that goes back to ancient times, to one of the first skepticss, named Sextus Empiricus. Sextus' problem is called the problem of the criterion. You will soon see why I'm raising this problem now. It goes like this. Suppose I want to say that a proposition, P, is true. But in order to claim that P is true, I have to have a criterion of truth. What is a criterion of truth? Well, it's something that will let us determine whether a given claim is true or false. An example of a criterion of truth would be: If I very clearly and distinctly understand that P, then P is true. (That was actually a criterion that Descartes gave, but I'm only telling it to you to give you an example.)
So suppose I've said that P, and I've offered some criterion, call the criterion C, that tells me that P is true. So what more does Sextus want? He says: but look, your criterion, C, is another claim. And so it, too, can be judged true or false only by a criterion. And look, if I apply the criterion to itself, I'm arguing in a circle; I'm begging the question. That is, if I say that C is true because it meets the criterion C, then I have presupposed just what I was trying to prove. And what good is that? That's begging the question, which is off-limits. So, Sextus said, you'd have to use some other criterion -- criterion D, say. But then D is yet another claim, and you need some other criterion to judge it. Like criterion E. Well, you can see where this is going. It's another regress and it looks like it can't stop.
Well, let's go back to the original claim, that P, and the original criterion that we used to judge it, namely C. Maybe we could say: a bunch of propositions are all known to be true; since criterion C just summarizes why they're all true. They are all very clearly and distinctly understood, say; so criterion C says, "Whatever is very clearly and distinctly understood is true." Then we have argued for criterion C without using another criterion -- we've just used a lot of individual claims.
I think you might very rightly find this confusing. If criterion C does summarize why a lot of individual claims are true, how does that show that we still don't need a criterion to know that C is true? In fact here's a good idea of what that particular criterion might be: "If a criterion summarizes why a lot of propositions are true, then the criterion is true." That's a criterion of truth that applies to criteria of truth!
It looks like there's no way out of using a criterion of truth, if we ever want to argue for a criterion of truth! And if Sextus Empiricus is right, we always have to use a criterion of truth if we want to be justified in saying that a particular claim is true. But then we are faced with circularity or regress. Either we argue in a circle and say that a criterion applies to itself, or we appeal to an infinite regress of criteria.
Now this view is, I think, philosophical poison. It's really insidious. If you think you must have a criterion, or some sort of standard that you use to judge claims true, in order to be justified in saying something is true, then you're going to end up facing skepticism. I want to give you what I, and quite a few other philosophers regard as the antidote to this poison. The antidote is due to two philosophers we've encountered before: Thomas Reid, and G. E. Moore. And this is called the method of common sense.
Reid pointed out that whenever we investigate anything at all, whenever we start thinking about some subject, we have to make assumptions. There isn't any way to avoid that. If you try to support your assumptions with reasons, you're going to end up assuming something else. So since we know in advance that we are going to have to make some assumptions, whenever we do philosophy, then what we have to do is clear: we have to assume those things that are most obvious, the matters of common sense that no one ever seriously doubts.
Now bear in mind, by "common sense" I don't mean old adages like "Chicken soup is good for colds." I mean much, much more basic claims than that. I mean claims like: "Human beings typically have two eyes, two ears, two hands, two feet," and so on. Or: "The world has a ground and a sky." Or: "Plants and animals come in a wide variety of sizes and colors." Or: "2+2=4." Or: "I am conscious and alive right now." These are all the absolutely most obvious sorts of claims that one could possibly make; and, said Reid and Moore, these are the claims that make up common sense. They are, you might say, the principles of common sense. And once again, the point is that since we know we are going to have to make some assumptions, then we might as well assume what is most obvious, namely the principles of common sense.
This is a very, very powerful thought, because it has the potential to undercut a lot of philosophical silliness and nonsense. Some people who have said, for example, that time does not really exist; it is ultimately only a complex illusion. I'm not making that up, some philosophers have actually said that, that time is an illusion. (This argument states that intrinsic to time is change. This illusion is relevant to the perception of animals and occurs in only one direction, namely that the present follows the past, and the future follows the present. It is quite possible for other cognitive forms to perceive change in different directions. However a "God's eye view" is capable of seeing all eternally [having no beginning or end] in an instant. By eliminating change one thus eliminates time.) Now, what does common sense have to say about that? Basically, that we can reject that theory out of hand. We scarcely need to argue against it: because any sophisticated philosophical premises that you would use in order to establish that time does not exist, are not nearly as obviously true, and as legitimately assumable, as the very claim in question, namely, that time does indeed exist. In other words, if we have to make a choice between, on the one hand, some rather dubious, sophisticated assumptions, that you'd have to make in order show that time doesn't exist; and, on the other hand, a principle of common sense, that time exists; if we have to make that choice, then there's no contest. We go with common sense and reject sophisticated silliness every time. (It might help to view this as a case of practicality, can one actually live their life as if time didn't exist?) Reid and Moore both approached a lot of problems in this manner.
Now Moore is famous for applying common sense to skepticism. I won't go into his argument in any detail, but I'll bet you can guess how it goes. Very roughly, he said: "I know that this is a hand; and I know that this is another; therefore I know that two things exist in the external world; therefore I know that there is an external world." That's the gist of his argument. How does that answer the skeptic, though, you might ask? Couldn't Moore be dreaming that he was seeing his hand? Couldn't he be seeing mere hand sense-data and no actual hand at all? No, said Moore: those possibilities may be ruled out because it's as obvious as anything can be that I know that this is a hand. That's a principle of common sense. It can't be rejected, certainly not based on any sort of sophisticated philosophical assumptions. So definitely, Moore thought, he is permitted to assume, without argument, that he knows that he has two hands; and he is permitted to use such ordinary, everyday assumptions to reject sophisticated philosophical theories, such as skepticism about the external world.
I don't know about you, but I find the method of common sense refreshing. I would be remiss if I did not mention, however, that some philosophers regard it as na´ve. But believe me, there have been some extremely intelligent, well-respected philosophers who have given some very sophisticated defenses of the method of common sense.
Now, I said that the method of common sense is the "antidote" to the problem of the criterion. How so? Well, like this: if the method of common sense is correct, then in philosophy, as in every day life, we may take the principles of common sense for granted, without argument. And we do not need criteria, not always, in order to judge whether a proposition is true or not. And for that matter, we can also take some criteria of truth for granted, according to common sense. Because definitely, some criteria of truth are principles of common sense. I mean that they are really obviously true.
Here's an example: If it seems to me that I am seeing myself in the mirror, and I have no serious reason to doubt this, then it's true that I'm seeing myself in the mirror. I claim that that's a principle of common sense: it is one of the sorts of things that we all take for granted, and never seriously in doubt; and if we do doubt it, the doubt is bound to seem just absurd. I could give lots of other examples, but I don't think that will be necessary.
Anyway, then, if the method of common sense is right, we can start our philosophical investigations with common sense, making particular claims about what is true, and also making generalizations, or stating criteria, about when claims are true. Because both the truth-claims and the criteria-claims can both be principles of common sense. Moreover, it's a principle of common sense that our basic faculties are reliable. Armed with those assumptions, that way, we can immediately reply to various kinds of skepticism, without wasting time thinking about skeptical doubts about dreaming and such. And that way, too, we can get around Sextus' problem of the criterion: there is no infinite regress or circle of criteria, because the buck stops with the principles of common sense.