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So last time we looked at scepticism about induction, this time we're moving on to scepticism about the external world,

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which is most famously exemplified in Descartes,

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were particularly the first meditation and is closely bound up there with related claims about the mind and body.

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So we're going to be looking at those two topics. Next time, we'll be moving on to more modern responses to scepticism.

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And in that connexion, discussing the topic of knowledge. Well, one can distinguish between two different types of scepticism.

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This is very rough and crude. Vertical scepticism, as it's sometimes called, is when you infer from one kind of thing to a different kind of thing.

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So, for example, I have perceptions of the world around me.

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I see colours, lights. I hear things, et cetera.

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And I infer that there is something out there.

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Objects, other people distinct from my perceptions.

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So it's called gold vertical because I'm, as it were, moving down a level or upper level,

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depending on how you look at it from one kind of thing to another.

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Now, distinguish that from horizontal scepticism where you're inferring more of the same.

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So scepticism about induction is an example of horizontal scepticism.

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I experience billiard balls bashing into each other, making each other move or apparently making each other move.

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I've experienced that in the past and I in for more of the same in the future.

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Now, you might naturally think that the vertical scepticism,

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such a scepticism about the external world is far more worrying than horizontal scepticism

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because it seems to put a whole category of things into doubt with the external world scepticism.

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I'm doubtful that there is anything beyond my perceptions. Maybe I am all there is.

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Me and my perceptions and all the rest is an illusion. So that can seem quite worrying.

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Descartes fleshes out this worry with various hypotheses,

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in particular the hypothesis that maybe he's dreaming everything and then he strengthens that even more.

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Maybe there's an evil demon causing me to have all these illusions carefully orchestrating everything I

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perceive in such a way that it seems that there's an external world when really it's all an illusion.

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And a modern variant of that is brain in a VAT scepticism.

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The idea that I might have been taken by some evil scientist or I may indeed have been created by this evil scientist,

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really there's just a brain somewhere in a vat and this wicked scientist is manipulating

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the inputs to my brain in such a way that I seem to perceive all this array of stuff.

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And in fact, none of it exists at all. Now, very quickly, Descartes way of dealing with scepticism is quite an extreme one,

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he says to start with, we shouldn't assent to anything that is less than completely certain.

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So he sets a very, very high threshold for knowledge. He's only going to count something as known or even legitimately believed in a sense.

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If it is completely certain and he casts around for it.

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Where can he find this certainty? And famously, he finds it in his own existence.

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I think therefore I am cogito ergo sum in Latin, very famous one, perhaps one of the most famous quotations in any philosophical works, at least here.

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It's impossible for me to be mistaken because even if I'm dreaming, I must exist.

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If an evil demon is implanting illusions into my mind, I must exist.

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If I'm a brain in a vat, I must exist. So all of these sceptical hypotheses actually imply my thinking and my existence.

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So therefore, I can at least be certain of those.

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Then very controversially, Descartes says, okay, I've got one bit of certainty here that I'm thinking that I exist.

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Can't I look at that instance of certainty and draw a general rule from it?

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What is it that makes me certain of my own existence? What is it?

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Well, I just clearly and distinctly perceive it to be true. Okay.

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Well, in that case, if clearly and distinctly perceiving it to be true is enough to make me certain in this case,

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then I can apply that as a general rule. Anything that I clearly and distinctly perceived to be true is true, or at least so it seems for now.

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I noticed that that move is a very, very dubious one. First of all, Descartes trumpets the cognito.

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I think, therefore I am as this special, unique truth that has this wonderful feature that all of the sceptical hypotheses imply it's truth.

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If I'm dreaming, I must exist. If I'm being deceived, I must exist.

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So it looks like the cognito is really special. But then he tries to draw a general rule from it, a rule that can be applied to other propositions.

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Anyway, he moves on with this general rule. But anything I clearly and distinctly perceive is true.

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He then says, I see that I have within me an idea of a perfect being, an idea of God,

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and I clearly and distinctly perceive that any perfection in that idea must come from a really perfect cause.

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And after various moves, he concludes, predictably, that God exists because only God provides an adequate cause of that idea.

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And a perfect God cannot deceive.

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Deceit is an imperfection. So having proved that God exists, I can be quite sure that my faculties are essentially reliable.

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That indeed anything I clearly and distinctly perceive when I use my faculties properly is true.

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Now, there's a very famous problem with this procedure known as the Cartesian Circle.

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And I want to draw your attention to the fact that it's not only a problem for Descartes.

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The interest in the Cartesian circle, the reason why it's discussed so much by scholars.

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He's not just some antiquarian interest in Descartes arguments.

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It's a much more general point.

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In Descartes case, he tries to prove the existence of God by relying on his clear and distinct perception, his mental faculties.

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He then appeals to the existence of a perfect God and non deceiving God to justify reliance on his mental faculties.

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And that just looks viciously circular. If he can trust his faculties to start with, then it seems that he doesn't need God to validate them.

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And if he can't trust his faculties to start with,

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then how can he justify the argument by which he reaches the conclusion that God exists and that his faculties are reliable?

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So either way, it looks like he's stuck. How can any anti sceptical argument even get off the ground?

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So, as I say, notice that this is a general problem for scepticism if or for attempting to produce an answer to scepticism.

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If you start by doubting your faculties, how on earth can you dig yourself out of that sceptical pit?

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Because he looks as though you've thrown away the tools that you might use for the job.

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If you can't rely on your faculties, then you've got nothing with which to construct any sort of argument against the sceptic.