‘But Descartes tried to work forward from this zero point. He doubted everything, and that was the only thing he was certain of. But now something struck him: one thing had to be true, and that was that he doubted. When he doubted, he had to be thinking, and because he was thinking, it had to be certain that he was a thinking being. Or, as he himself expressed it: Cogito, ergo sum.’ ‘Which means?’ ‘I think, therefore I am.’
