MARY: ‘Cogito ergo sum’ – I think therefore I am – Descartes' famous proof that he exist!

JOHN: I don't see that Descartes has proved anything.

MARY: Why not?

JOHN: Well, before Descartes arrives at his ‘proof’, he sets about trying to subject all his beliefs to doubt, right?

MARY: Yes. That's his famous ‘method of doubt’. He decides to set aside all of those beliefs which he can doubt, to see if he find any indubitable certainties. He wants to identify those of his beliefs which are utterly secure, so that he can then rebuild other knowledge upon them. They will provide him with a firm foundation. And the cogito is his starting point – Descartes cannot doubt that he exists because, by doubting he exists, he proves that he does. It's a self-defeating doubt.

JOHN: Hmm. I am not so sure. Look, Descartes supposes at one point that there might be an evil demon intent on deceiving him – correct?

MARY: Yes. Descartes doesn't actually believe there is such a demon, of course. But he supposes, for the sake of argument, that there might be. A demon so powerful that it could cause Descartes to have deceptive experiences, deluding Descartes into supposing he inhabits a real, physical world of trees and houses and other people, when in truth it is all an illusion.

JOHN: Right. Descartes ask himself: how can I know that I am not being deceived by such a demon? After all, everything would seem just the same. And if I can't tell that I am not, then I cannot know that I am not.

MARY: But then Descartes realizes that, while such a powerful and deceitful demon might deceive him about there being a physical world, it could not deceive him into thinking he exists when he doesn't. Not even such a demon could deceive Descartes about that.

JOHN: So you say. But look, Descartes admits that such a demon might mess with Descartes' mind in other ways too. For example, it could make Descartes go wrong in his reasoning.

MARY: Yes, that's true.

JOHN: But then Descartes ‘proof’ must be useless. Notice the word ‘ergo’ – therefore. Descartes has constructed a piece of reasoning, from the premise that he thinks to the conclusion that he exists. Correct?

MARY: It looks that way, yes.

JOHN: But then how does Descartes know that this little argument – I think, therefore, I am – is sound? Perhaps the demon is deluding Descartes into thinking it's sound, when it's not. You see? Descartes' proof is a piece of reasoning, and Descartes cannot yet trust any of his reasoning, because there demon might be making him go wrong.

MARY: Okay, I see the problem. Or rather I see what you think the problem is for Descartes. In fact, I think you have misunderstood him.

JOHN: How?

MARY: ‘Cogito ergo sum’ looks like an argument, a piece of reasoning, true. But it's not. It is something else. It is a performance.

JOHN: A performance? In what sense.

MARY: By doing some thinking – some doubting – Descartes shows that he exists. He doesn't infer he exists. He actually demonstrates it, by thinking.

JOHN: So the cogito isn't an argument after all? An argument from the premise ‘I think’ to the conclusion ‘I am’?

MARY: Correct. If it were, then Descartes could not be sure an evil demon had not made his reasoning go wrong. But the cogito is actually a performance. Descartes proves he exists by engaging in an activity – by thinking, he demonstrates he exists.

JOHN: But wouldn't another activity do just as well? Riding a bike, say? Wouldn't that demonstrate he exists?

MARY: No, because Descartes couldn't be sure he really was riding a bike. The evil demon might delude Descartes into thinking he's riding a bike, when he's not. But the demon cannot make Descartes think he's thinking when he's not.

JOHN: Hmm. I am still not sure Descartes has proved beyond all possible doubt that he exists. Let me think about it some more…