René Descartes, answered from the texts and cited to the page.
The dream argument begins from a fact every sleeping person knows: that in the night, one finds oneself convinced of being awake, seated by the fire, dressed and occupied with familiar things, when in truth one lies undressed in bed.1 This is not a rare aberration but a habitual occurrence, and the question it presses upon the waking philosopher is severe — by what certain mark do we distinguish the waking state from the dreaming one?
When I attend carefully to this reflection, I find that no such mark presents itself with certainty, and the astonishment this produces is very nearly sufficient to persuade me that I dream even now.2 The argument does not stop at the senses. Even granting that the particular things of waking life — this hand, this head, this fire — might be illusions, the dreaming mind still represents something; and those representations, like the paintings of artists who compose sirens and satyrs from parts of real animals, must ultimately rest on simpler elements that are real.3
A painter who invents the most fantastical creature cannot invent new colours; he can only recombine what already exists. So even in dreams, there are certain simple and universal natures — extension, figure, number, magnitude — which are not themselves called into doubt by the dream supposition. But the argument can be pressed further still.
Even the demonstrations of mathematics, which seem quite independent of whether I am asleep or awake, fall under suspicion — for we remain ignorant of whether God may not have created us so as to be deceived even in what we believe we know most certainly.4 And if we suppose some less perfect author of our being rather than an omnipotent God, that only strengthens the suspicion, since the less perfect the source, the less reason to trust what it produces.
What survives all of this is the point at which the doubt cancels itself: I who doubt these things cannot at the same moment be conceived as not existing, for there is a plain contradiction in supposing that what thinks does not, in the very act of thinking, exist.5 The dream argument is thus not a counsel of despair but the instrument by which the one indubitable foundation is reached. It clears the ground; the cogito is what stands on it.
How often has it happened to me that in the night I dreamt that I found myself in this particular place, that I was dressed and seated near the fire, whilst in reality I was lying undressed in bed!The Philosophical Works of Descartes Vol I
in dwelling carefully on this reflection I see so manifestly that there are no certain indications by which we may clearly distinguish wakefulness from sleep that I am lost in astonishment. And my astonishment is such that it is almost capable of persuading me that I now dream.The Philosophical Works of Descartes Vol I
painters, even when they study with the greatest skill to represent sirens and satyrs by forms the most strange and extraordinary, cannot give them natures which are entirely new, but merely make a certain medley of the members of different animalsThe Philosophical Works of Descartes Vol I
we have been told that God who created us can do all that He desires. For we are still ignorant of whether He may not have desired to create us in such a way that we shall always be deceived, even in the things that we believe ourselves to know bestThe Philosophical Works of Descartes Vol I
we cannot in the same way conceive that we who doubt these things are not; for there is a contradiction in conceiving that what thinks does not at the same time as it thinks, exist.The Philosophical Works of Descartes Vol I