The next move he made led to one of the best-known lines in philosophy, though many more people know the quotation than understand what it means. Descartes saw that even if the demon existed and was tricking him, there must be something that the demon was tricking. As long as he was having a thought at all, he, Descartes, must exist. The demon couldn't make him believe that he existed if he didn't. That's because something that doesn't exist can't have thoughts. ‘I think, therefore I am’ (cogito ergo sum in Latin) was Descartes' conclusion. I'm thinking, so I must exist. Try it for yourself.
...more
