The dream and the evil demon
Descartes · Meditations on First Philosophy · 1641
You have surely dreamed a scene so vivid that, in the moment, nothing set it apart from reality. But then, what assures you that at this very instant you are not dreaming? Descartes presses the test further: imagine an all-powerful, deceiving mind, an evil demon, bending all its cunning to make you take the false for true, down to the sky, the earth, and your own body. As long as this hypothesis is not ruled out, almost all your beliefs could be false without your being able to know it.
Faced with this doubt, what remains to you? Does anything withstand the test, or must we admit that no definitive certainty is within our reach?