This idea of wholesale radical deception has been the subject of popular films such as The Matrix and Twelve Monkeys. Descartes realises, however, that there is one proposition that neither the evil demon nor even God could ever make false. This is that at any time when he thinks, it must be the case that he exists. For he must exist in order to be able to think. By such reasoning Descartes is led to the cogito as the one certain, infallible rock of knowledge.
