When we’re focused on the search for truth, we’ll begin by doubting the existence of the objects of sense-perception and imagination. There are two reasons for this: (1) We have occasionally found our senses to be in error, and it’s not wise to place much trust in anyone or anything that has deceived us even once. (2) In our sleep we regularly seem to see or imagine things that don’t exist anywhere; and while we are doubting there seem to be no absolutely reliable criteria to distinguish being asleep from being awake.

— Rene Descartes, The Principles of Philosophy