Alan Saunders: Hi, I'm Alan Saunders and you're listening to The Philosopher's Zone. Or are you? It's possible that you're mistaken, and there are various ways in which you might be mistaken. You might have mis-heard my opening words, so you might be listening to another program entirely.

Or you might be asleep and just dreaming that you're listening to me (and what a dream that would be). Then again, perhaps there is no me and there are no radios and you are just a disembodied brain lying in a vat somewhere which scientific deceivers who might be aliens from another world, feeding you with impressions that you think of as life and the real world. In other words, you're like a character in the movie The Matrix.

While we're at it, how do you know that it was me you were listening to about a minute back when I said who I was? Perhaps the whole world came into being 30 seconds ago, complete with you and what you think of as your memories. And even if you're sure that the past really is the past, what about the future? Is the fact that life has gone on in more or less the same way for ever in the past, the sun rising every morning, summer following spring, any reason at all for thinking that things will carry on like this forever in the future?

And is it worth asking these questions, especially as most of them cannot be definitively answered. Well, Stephen Hetherington, Professor of Philosophy at the University of New South Wales, thinks that scepticism, which is what we're talking about, is a valuable exercise. We should question the very fundamentals of our claims to knowledge about the world, because if we don't question them, we won't really know what they amount to.

His book, Yes, But how do you Know? is an introduction to philosophy through sceptical ideas. He begins by quoting himself to the effect that sceptical thinking is one of the most authentically philosophical ways of thinking there are. Why is this?

Stephen Hetherington: Oh, it's true, I did quote that; that was when I was early in my career, but yes, I still think that. Obviously I don't think it's the only aspect to philosophy, but the idea was, look, it's just so easy to take stuff for granted, and if philosophy is nothing without basic core motivating ideas, and I guess I've always thought one of the basic philosophical ideas, and not only that, an impulse, an instinct, is instinct to question, not to question blindly, to question with an idea, with your eye on truth, and try to look beyond appearances.

So if you think for example metaphysics traditionally as thinking about reality beyond appearances, then you think epistemology at its most basic, is going to be thinking about knowing reality in part, potentially beyond appearances. And so when you put it like that, you know, in a few sentences, I'm already starting to sound philosophical.

Alan Saunders: And you then move on to what you call 'a thousand dollar doubt'. What's a thousand dollar doubt and why's it worth a thousand dollars?

Stephen Hetherington: With inflation. Well the idea there was it's easier to sort of say, Well I'm not really sure about something, beliefs are easy, you can believe anything. Well, try it. Even if I offer you a thousand dollars, I don't think you can make yourself believe. So the idea is you look out the window, it's raining, believe it's not. Here's a thousand dollars if you can genuinely believe that you're not. Not just say it.

Alan Saunders: Isn't that though just a psychological fact about me that I can't believe it? It doesn't follow from my inability to enter into state of mind where I don't believe it's raining, it doesn't follow from that that it actually is raining.

Stephen Hetherington: No, quite. So the idea is that sort of question is used as just a way to sort of lead someone in to the idea of thinking what possible ideas or ways of thinking, could - and then of course the critical move - should, get you to change your beliefs. So at first if you say, 'My gosh, here's $1,000 can you change your mind?' 'Well no, I can't because there's the evidence.' 'Ah, evidence', and with that suddenly we're introducing a normative element. Because with the evidence, the idea is therefore we should have this or that belief. And once we're starting to think like that, we're starting to do epistemology. And you know, your sceptical doubts are coming along down the line. But that's the first move.

Alan Saunders: One of the important aspects of scepticism is that it doesn't actually have to give us a reason for specific doubts about what we claim to believe. It can just raise the possibility. Is it, for example, possible that I am not seeing the world, I'm not seeing the rain, but that I'm just dreaming it?

Stephen Hetherington: This is a possibility that famously Descartes considered. He wasn't actually the first to consider that one, it cropped up in the Theaetetus as well, one of the famous Platonic dialogues. But Plato didn't do as much with it. Descartes had it highlighted there in the First Meditation as seemingly a core element in what for many has been the most pronounced episode of sceptical doubt in modern philosophy. So is it possible? Well the Sceptic would say so. The Sceptic doesn't believe that it's dreaming, and what you're asking is Is the Sceptic doing anything more than just sort of mentioning an enticing odd little possibility which if we're going to be open-minded and fair and intelligent, we suddenly think is possible. And having admitted that it's possible, questions can be ruled out. And the Sceptic of course is saying 'Well you can't'. And if you were dreaming, you wouldn't be perceiving.

Alan Saunders: No. So this is a version of scepticism about the external world.

Stephen Hetherington: Correct. And so in fact the idea of the external world, we get that idea in part through the sceptic of thinking. Because in other words, if you don't even sort of think of the possibility of you undergoing subjectively felt experiences which could nonetheless be radically misleading about what they seem to be about, then if you don't open yourself to that, then you mightn't even think of a world beyond. But once you do think of those experiences, that are potentially misleading, then you can say for example what are they misleading about? And you'd say, Well, it's about a physical world. You could even then though raise a more radical vow and say could there be no physical world?

Alan Saunders: Well I suppose one answer might be, Well, what is this external world? There is in fact no world external to me, I am not trapped in my own mind, looking out. I am part of the world, and I cannot help but trust my perceptions of it.

Stephen Hetherington: Look, I've got to say I actually am not a sceptic, I agree with you, seriously, but the idea is, as I said, if you're going to reach your sort of positive optimistic gung-ho view of ourselves as functioning in the world, as knowing beings, etc., the idea of the motivation behind this kind of approach to introducing people of philosophy is just to say Look, don't at least reach for that positive view too quickly. Stop, take a detour, maybe you'll get lost, and maybe you won't come back. But at least think about it and tell us why you've got that knowledge. That's the critical thing with the sceptic. And taking sceptical ideas seriously, you don't have to be saying 'I genuinely give up on the idea of the world' for example, it's just the very least it's saying, 'If I end up where I started, I'll be different.

Alan Saunders: The opposite of the position I've just mentioned, where I refuse to accept that there is such a thing as a world external to me, is the possibility that I am in fact just a brain in a vat. This is, as it were, The Matrix hypothesis. Tell us about the importance of the brain in the vat.

Stephen Hetherington: It's a modern version of a similar idea. Occasionally people treat it just as basically Descartes' dreaming argument, but made more catchy in a modern way. It's also got elements of Descartes' evil genius argument in there as well. The evil demon possibility was just the idea that any idea you have could be somehow in your mind because there is this evil demon. For Descartes it's something with godlike powers but directed only at messing around with your mind.

OK, well what's a brain in the vat possibility? You've been asleep, and during your sleep, your brain has been carefully and silently abstracted from you, put into an appropriate vat, all kind of good chemicals in there, hooked up to the machine, and why do I mention evil genius? Because this is overseen by malevolent scientists. Well I don't know if they're completely malevolent, but scientists who want to mess around with your brain.

And here's the trick, the way they manipulate your brain, it's for you, subjectively, just as if you're still living your normal life. You notice no difference. Are you therefore completely lacking knowledge of an external world in the physical world around you? Do you not know that there are waves of acid wafting around your brain? No, you don't, according to the argument anyway.

Alan Saunders: Well I suppose a slightly radical response to the brain in the vat hypothesis, and also to the matrix hypothesis, which is very similar to the brain in the vat, is that it's not as though I'm being misled about reality, that is reality, it's a different level of reality, what I'm getting put in my brain, while I'm in the vat.

Stephen Hetherington: What do you mean, you're thinking like, suppose what's being fed to you is an image as of a tree, but you're inside a room, there are no trees there. To you it feels as if you're looking at a tree.

Alan Saunders: I can pick apples from it, I can feel the roughness of the bark, I can hear the leaves whistling in the wind, what more do you want?

Stephen Hetherington: All right. Two things there. One, of course the obvious answer there is you're doing none of those things. Every description you gave is you can put in front of it 'it's as if'. It's as if you're hearing the wind, it's as if you're picking apples, etc. So then the second point is why does it matter?

Now obviously one of the traditional answers is Well in fact you're just wrong, and of course a guiding aim in thinking about knowledge, which is what we're doing, is that the desire to explain how we get the world right.

Alan Saunders: Suppose the world outside the vat does not contain any trees. Then my awareness or apparent awareness of a tree in my capacity as a brain in a vat, that's what being aware of a tree is, isn't it? Because that's the only place where there are trees, my brain.

Stephen Hetherington: Well do you want to be an idealist in the world?

Alan Saunders: I was about to come on to idealism. Let's come on to idealism. The idea is that I can know the physical world by way of ideas simply because the physical world is entirely made of ideas, that's all there is.

Stephen Hetherington: Yes, well that's true. True as in there's a philosophical response. It's another in the garden of philosophical responses, and yeah, look, obviously who you're thinking of there is Berkeley, Bishop George Berkeley,

Alan Saunders: The 18th century Irish philosopher, yes.

Stephen Hetherington: Exactly. In fact the first person who got me absolutely excited about philosophy, way, way, back. So, brilliant. But that said, for most philosophers it feels like a rather, well, drastic, extreme response. Look, I mean that obviously doesn't disprove it, but the idea is Look, what you're getting at is that in thinking about, starting to think about some odd possibilities, you know, sceptical possibilities there, we're not exactly free but we're finding ourselves reaching for some quite dramatic metaphysical pictures.

And of course again, that's part of the strength, the danger but also the strength, of taking sceptical ideas seriously. Because they really go to the heart of basic ways of philosophical thinking. You can mess around in philosophy with certain things and you will not have to come to grips with something as basic, as in central, as what you've just raised. But you do, with sceptical stuff, yes.

Alan Saunders: Well I suppose what I really want and this involves invoking another 18th philosopher Immanuel Kant, is to know things in themselves, and not merely as appearances or sensory input. Now Kant didn't think that was on, did he?

Stephen Hetherington: No. And of course how to interpret that; is that again, a metaphysical result about the nature of reality that there are things in themselves, but there's a limit to how many of them? Or do we take another approach and say for example that because even the idea of our knowing about something, completely as it is in itself independently of any kind of potentially knowing interaction we have with it, because that seems impossible, let's suppose, then the idea of there being such an aspect of the world, will be beyond us. So notice again just how incredibly central a skeptical argument or the sceptical idea has become. You're right. I mean there's no-one more important in the history of modern philosophy than Kant.

Alan Saunders: Backing the truck up a bit, couldn't I say that the evidence that my perceptions of the world are valid, doesn't have to amount to an absolutely solid, knock-down proof.

Stephen Hetherington: Yes, look I'm completely with you on this. Right, so look, we have a couple of questions here about how to interpret sceptical thinking. Right, so what we've had so far in the discussion is mention of some sceptical possibilities. Then what you have to think about with thinking that scepticism is the kind of standard that the sceptic is expecting you to meet.

For example, suppose you said something like, Yes, I accept that I should know that I'm not dreaming and you know what, it's not that hard. Well as you're aware the sceptic traditionally is going to come back to you and say Hang on, it's really hard, and you're just begging the question against me if you say that it's not hard, and you make do with lower standards.

Isn't, he or she will say, isn't one of the lessons of sceptical thinking precisely the idea that just as we have to take seriously odd possibilities, we also have to hold ourselves to really high standards, and so when we talk about knowledge, we really talk about 'knowledge'.

Alan Saunders: Getting back to Rene Descartes back in the 17th century, he claimed that the one thing he couldn't doubt is (he's doubting everything), but he says the one thing that he couldn't doubt was his own existence, and that the assertion of his own existence must be true whenever he conceived or uttered it. And this is a version of the argument that is known as the cogito ergo sum, I think therefore I am. Isn't there a problem here in that I can doubt my own existence, I just can't coherently doubt it. It's incoherent for me to say that I don't exist, but what evidence do we have that things are, or need to be coherent?

Stephen Hetherington: This is again one of the interesting things about this kind of move. Because I mean notice the very first question, we started talking about the $1,000 doubts, then of course as you rightly say, Well, we could or couldn't doubt, as just a psychological, but fact the in the face of whatever evidence is worth, should you doubt? And that's what your asking is the same kind of idea again. Would it be coherent for him to doubt, is it a rational doubt, and that's what we're talking about here.

So of course which then leads us to think Well, what's the scope of rationality? What counts as a rational doubt? So for example, even asking more generally, not just Descartes, but if someone says Look, here's the following possibility, we could all be wrong in the following way, blah-blah-blah. And you say, Well that doesn't make sense. And it might not in a particular case, but then we think the dreaming possibility or Descartes' evil demon possibility, we can just about sort of understand that as coherent. But how? It's true, we don't have absolutely simple, agreed-on criteria of coherence. You know, Descartes talked a lot about clarity and distinctness of ideas, but we tend not to do it. I think it's actually a bit vaguer now, it's more like Gee, well if we can kind of describe it, if we can kind of think of, I don't know, if we can imagine it somehow....

Alan Saunders: Of course, scepticism doesn't have to be about the present. It can be about the future, and it can be about the past. Let's take the past first, and this is something I remember asking myself when I was a kid, how do I know that the entire universe didn't come into existence five minutes ago, including me and all that I think of as my memories? I suppose that's a version of the dream hypothesis, but again, it seems to be an unanswerable sceptical question.

Stephen Hetherington: Well, how old were you, by the way?

Alan Saunders: I don't know how old I was, I just remember. Or I think I remember.

Stephen Hetherington: Yes, did you come up with an answer at the time or did you just take it as an imponderable?

Alan Saunders: I think I took it as an imponderable.

Stephen Hetherington: Yes. Look I've got no simple answer, but yes, it's a similar kind of idea, and you know, it's really interesting, because when I teach, you can also set this argument up in terms of Humean doubts.

Alan Saunders: This is appertaining to David Hume, the 18th century Scottish philosopher.

Stephen Hetherington: Yes. And so the idea is for example, because you can think of it in terms of Well look, you've got the evidence that's right in front of you, you've got the observations, just suppose for the moment even if we grant you your observations, even if we put to one side say the dreamy argument, then we grant you that yo u're OK with what you're looking around at you at. Nonetheless, you're making various assumptions about how the world has been in the past, to get you to this position.

You're saying for example you're relying on memory, memory could be misleading you. So the idea is, how do you extrapolate from the present observations to thinking about say, how the world has been in some bigger way in the past, that you didn't directly observe? With Hume it was this sort of regularity idea, the idea that the world hasn't suddenly been playing tricks on us, and we can extrapolate from how things are now to how things have been in the past.

But I was going to say what's interesting when I teach Hume, is that students are very OK with the idea that there's a sort of puzzle about knowing the future. But they don't feel it, in my experience they don't feel it as much about the past. So it's part of why I asked you how old you were when you felt that.

Alan Saunders: Well let's look at the future. How do I know that the sun will rise tomorrow? It has risen every day so far, as far as I know, but can I be really sure that it will do so tomorrow? And does the fact that it has risen innumerable times in the past, entitle me to expect confidently that it will rise tomorrow?

Stephen Hetherington: Well look, this is one of the classic instances or applications of Hume's, what's these days is called his argument for inductive scepticism. Look, I'll say one thing, that's exactly a way to present Hume, but you know, you can make Hume's argument much more powerful still because it's not even just about feeling certain or being sure. Again I mention the regularity kind of principle or assumption that you make. Suppose you said as you might, Look, I'll accept that it's not - I can't know for sure that the sun will rise tomorrow. Just suppose you said that. But then suppose you said, Oh, but look, I can be - it's very probable. I can be rationally very confident.

OK, but the Humean argument will say to you, But even that relies upon the idea that the experiences that you and others have had in the past, or the testimony you've had etc., about sunrises you haven't been present at, etc, all of that is a good guide to how things are going to be, even a probabilistic good guide. And that itself is still a regularity kind of assumption.

And the key part to the Humean argument is to say But how would you know that? Even probabilistically, how you're going to know it? You don't know it just by pure thought, pure go looking in the corner hard and thinking really hard, you're not going to know it. Again, the idea will be you've got to rely on observational evidence, even to know that the world is going to be regular in that way. Even in a probabilistic way, and that's critical to the argument. That's part of what I like, that's what I like about that argument, because again with students, that's often what they miss to start with, and so you've really got to get that through, and it's a great aspect to the argument. That's what makes it a really pressing argument.

Alan Saunders: In the classical world, there was a distinction between what's called academic scepticism and Pironian scepticism. The academic sceptics, the members of Plato's academy said that nothing could be known. Now the Pironians thought this was altogether too dogmatic, because it implied that there was one thing that could be known, namely that nothing could known. So what was the Pironian position?

Stephen Hetherington: Ah, well, the idea there was that for any kind of position you might want to adopt as to how things really are, there's a counter-position. And it's not just that we could make it up if we were really imaginative. In fact when they argued for it, they looked around and they listened to people who seemed to come up with different positions. It wasn't just other people, you know, you versus other animals, you today versus you yesterday, you yesterday, you today, etc. This society, that society, you well, you ill, all kind of states in which you could be or whatever, in which positions can be derived. They seem to balance, counter-balance each other. What are we to do rationally? We should suspend belief, the original beliefs, those beliefs as to how things really are. Just forget them.

And the idea was again they didn't say Ah, this is definitely how it is. It's more like Oh, we find ourselves suspending belief when we're confronted by this kind of input. And gee whiz, they said, this is kind of relaxing. The word that's used usually in translation is 'tranquil', 'tranquility'. So they thought that sceptical thinking gives you that kind of state.

Alan Saunders: So this is similar to the position adopted in the 16th century of Michel de Montaigne, who said, There are rival opinions, just live with it.

Stephen Hetherington: That's the view.

Alan Saunders: Stephen Hetherington, thank you very much for joining us.

Stephen Hetherington: Thanks very much, Alan.

Alan Saunders: Stephen Hetherington is Professor of Philosophy at the University of New South Wales.

For details of his book, Yes, but how do you know? Introducing philosophy through sceptical ideas, check out our website.

That's also the place to go if you've got something to say about this week's show. Just click on where it says 'Add your Comment'.

The show is produced by Kyla Slaven, the sound engineer is Charlie McKune, I'm Alan Saunders and I might be back next week, but who knows?