I will think that the sky, the air, the earth, colours, shapes, sounds, and all external things are no different from the illusions of our dreams, and that they are traps he has laid for my credulity; I will consider myself as having no hands, no eyes, no flesh, no blood, and no senses, but yet as falsely believing that I have all these.
I will therefore suppose that, not God, who is perfectly good and the source of truth, but some evil spirit, supremely powerful and cunning, has devoted all his efforts to deceiving me.
For whether I am waking or sleeping, two plus three equals five, and a square has no more than four sides; nor does it seem possible that such obvious truths could be affected by any suspicion that they are false.
When I think this over more carefully I see so clearly that waking can never be distinguished from sleep by any conclusive indications that I am stupefied; and this very stupor comes close to persuading me that I am asleep after all.
Certainly, up to now whatever I have accepted as fully true I have learned either from or by means of the senses: but I have discovered that they sometimes deceive us, and prudence dictates that we should never fully trust those who have deceived us even once.