The dream argument is the postulation that the act of dreaming provides preliminary evidence that the senses we trust to distinguish reality from illusion should not be fully trusted, and therefore any state that is dependent on our senses should at the very least be carefully examined and rigorously tested to determine if it is in fact reality.
While people dream, they usually do not realize they are dreaming. 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). Having received serious attention in René Descartes’ Meditations on First Philosophy, the dream argument has become one of the most popular skeptical hypotheses.
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 Carroll’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.”