While people dream, they usually do not realize they are dreaming (in non-lucid dreams). This has led philosophers to wonder whether one could actually be dreaming constantly, instead of being in waking reality (or at least that one can't be certain that he or she is not dreaming). In the West, the philosophical puzzle is referred to as early as Plato (Theaetetus 158b-d) and Aristotle (Metaphysics 1011a6). Having received serious attention in René Descartes' Meditations on First Philosophy, the dream argument has become one of the most popular skeptical hypotheses.
In the East, this type of argument is well known as "Zhuangzi dreamed he was a butterfly" (莊周夢蝶 Zhuāng Zhōu mèng dié) introduced by Zhuangzi. It relates that one night Zhuangzi dreamed that he was a carefree butterfly flying happily. After he woke up, he wondered how he could determine whether he was Zhuangzi who had just finished dreaming he was a butterfly, or a butterfly who had just started dreaming he was Zhuangzi. This was a metaphor for what he referred to as a "great dream."
He who dreams of drinking wine may weep when morning comes; he who dreams of weeping may in the morning go off to hunt. While he is dreaming he does not know it is a dream, and in his dream he may even try to interpret a dream. Only after he wakes does he know it was a dream. And someday there will be a great awakening when we know that this is all a great dream. Yet the stupid believe they are awake, busily and brightly assuming they understand things, calling this man ruler, that one herdsman ‑ how dense! Confucius and you are both dreaming! And when I say you are dreaming, I am dreaming, too. Words like these will be labeled the Supreme Swindle. Yet, after ten thousand generations, a great sage may appear who will know their meaning, and it will still be as though he appeared with astonishing speed.
Whatever I have accepted until now as most true has come to me through my senses. But occasionally I have found that they have deceived me, and it is unwise to trust completely those who have deceived us even once.|||René Descartes
Dreaming provides a springboard for those who question whether our own reality may be an illusion. The ability of the brain to trick itself into believing a neuronally generated world is the "real world" means at least one variety of simulated reality is a common, even nightly event.
Those who argue that the world is not simulated must concede that the mind — at least the sleeping mind — is not itself an entirely reliable mechanism for attempting to differentiate reality from illusion.
This could be seen as a challenge to those who claim a simulated reality requires highly advanced scientific technology, since, if dreaming really is a form of virtual reality, the only apparatus needed to construct a simulated reality capable of fooling the unconscious mind is a human brain.
In Lewis Carrol's Through the Looking-Glass, Alice finds the Red King asleep in the grass; Tweedledum and Tweedledee tell her that the Red King is dreaming about her, and that if he were to wake up she would "go out—bang!—just like a candle."
In the the Matrix series, people inside the Matrix itself are dreaming, but not knowing it. Neo, the hero of the movie, is rescued by a band of real people and embarks upon freeing the rest.