Descartes and the evil genius?
Descartes creates the evil genius in an attempt to throw everything that he thinks he knows into question.
The evil genius has god-like power that he uses to deceive Descartes. Descartes cannot disprove the existence of such a creature. You cannot trust your thoughts in such a scenario, thus Descartes cannot be certain that any of the information he gets from his senses, his memories, his thoughts, or his beliefs is real.