4.2 Possible Answers to Scepticism of the External World

So, it looks like if we want to answer skepticism, we might need to do so in a slightly different way, looking for a straightforward argument to justify knowledge against the skeptic. It looks intrinsically rather problematic now. G.E. Moore, a very famous Cambridge philosopher of the early 20th century, suggested the following refutation of skepticism: "Here's one hand, here's another hand. Well, if this is a hand, if this is a hand, then there's an external world because hands are physical objects. Yet, therefore, there is an external world and skepticism is refuted." Now, one's first reaction on coming across that argument is to think, "This is just ridiculous. Come on, I wanted a serious argument against the skeptic and all this chap is saying is here's one hand, here's another. There you are, hands exist, external objects exist. Surely that isn't any good." Well, I think one can look a little bit deeper into this and see that there may be a potentially valuable strategy here. Now, very well-known forms of argument are dignified with ancient Latin names. You will quite likely have come across at least the first of these, very commonly used: modus ponens. What modus ponens is, is the method of inference whereby you go from P implies Q and P is true to the conclusion that Q is true. That is clearly a valid method of inference. If P does imply Q, that just means that if P is true, Q is true too. So, if P is true, it follows that Q is true. But notice that there is another valid form of inference which is also connected with implication, and it's called modus tollens. P implies Q, Q is false, therefore P is false. Think about it for a moment, check you agree that is a valid form of inference. If P implies Q, that means if P is true, Q has to be as well. So, if Q is false, P can't be true. So, unlike the other two methods of inference, you can't say P implies Q, Q therefore P. That's invalid. And it's invalid to say P implies Q, not P, therefore not Q. But these two are both above reproach. Now, what that means is that if you have an implication P implies Q, you can't only argue forwards, you can also, as it were, argue backwards. So let me give you a practical example of this. If you look in the Bible, in chapter 20 of Deuteronomy, you will find that the Jews are supposedly commanded to annihilate six whole nations of people who have the misfortune to live in the cities that the Lord your God has given you for an inheritance, and you're told to save alive nothing that breathes, but utterly destroy them, not only the fighting men, but the women, children, animals, everything. Now, you can imagine a religious fundamentalist who argues like this: everything in the Bible is true; therefore, genocide is sometimes desirable. But you can imagine a philosopher arguing in the opposite way: genocide is never desirable; therefore, not everything in the Bible is true. Now, the point here, what this highlights is this: the two people who argue in these ways are obviously disagreeing, but we can focus that disagreement on the two underlined premises. They can agree about what the Bible says. One of them is starting from the premise that everything in the Bible is true. One of them is starting from the premise that genocide is never desirable. And you can ask yourself, well, which of those two do I hold more strongly? And I think most people would say the latter has a lot more to commend it than the former. And this is the sort of thing that could give pause to somebody who starts off from the premise that everything in the Bible is true. So, what I'm pointing out here is the fact that when we consider philosophical problems, we don't always start from given premises and argue forwards. Sometimes we look at where those premises lead us and then rethink our commitment to those premises. So, I can imagine lots of Christians might very well start from the hypothesis that everything in the Bible is true. You maybe, they're taught it when they're young and so on. Then, when they come across these things, they don't just argue forwards and say, "Yeah, okay, this follows." They actually go back and reconsider the premises from which they start. Now, let's take a look at Moore's argument in that light. If this is a hand, there is an external world. Okay, because if this is a hand, it's an external object and physical object and therefore there is an external world. So, that's agreed and Moore is effectively saying, "We know this is a hand, therefore we know there's an external world." Whereas the skeptic says, "We don't know that there's an external world, therefore we don't know that this is a hand." Think of that in the light of one person's modus ponens. One person wants to argue forwards, one person wants to argue backwards. Moore can perhaps plausibly claim that his premise, "We know that this is a hand," is more plausible than the premise of the skeptic, "We don't know that there's an external world." That, after all, is a very difficult theoretical claim, the claim that we do not know that there's an external world. The claim that we know this is a hand might seem, when you put the two against each other, to have more to commend it. So, in effect, what Moore is saying is that Descartes is wrong to put all the onus of proof on the person who wants to oppose the skeptic. The skeptic, too, is making a substantial claim, a substantial claim that we do not know certain things. Maybe that needs to be put against the claim of ordinary believers that they do know such things. Okay, well, probably most of us would like to agree with Moore. We'd like to think, "Yes, we do know that this is a hand." But the argument still seems rather crude. It looks as though we want some sort of philosophical argument rather than just a bare common-sense claim to justify knowing that this is a hand. But as I've said, the Cartesian arguments have great difficulty doing the job they seem to be trying to pull. Themselves up by their bootstraps. As I've said, if you try to justify your faculties using your faculties, it just looks circular. So, trying to get anything more substantial than Moore's argument does seem rather thin, or can do, certainly from many perspectives. It seems hard if we rely on the sort of strategy that Descartes tries. So, many recent philosophers have moved away from the Cartesian idea, that is, the idea of trying to argue from the inside, trying to prove everything using our faculties and what is immediately apparent to us, towards a position called externalism. And we'll be discussing that next time. And if you've covered induction at all and looked at Mellow's approach to induction (I mentioned it last time very briefly), you'll see that there's a close similarity there as well. Externalism has become a very popular way of addressing the skeptic. Okay, so I'm putting that on one side until we come back to knowledge.