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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 (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, first formally introduced by René Descartes in 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.^[1]

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."

An intriguing paradox concerning dreams and the nature of reality was described by the British writer Eric Bond Hutton in 1989.^[2] As a child Hutton often had lucid dreams in which people and things seemed as solid and real as in waking life. This led him to wonder whether life itself was a dream, even whether he existed only in somebody else's dream. Once in a while he would have a pre-lucid dream (in which one suspects that one is dreaming). He always found these somewhat disturbing, but one day hit upon a magic formula to be used in them: "If I find myself asking 'Am I dreaming?' it proves that I am, since this question would never occur to me in waking life." Yet, such is the nature of dreams, he could never recall it when he needed to. Many years later, when he came to write about his childhood fascination with dreams, he was struck by a contradiction in his earlier reasoning. True, asking oneself "Am I dreaming?" in a dream would seem to prove that one is. And yet that is precisely what he had often asked himself in waking life. Therein lay a paradox. What was he to conclude? That it does not prove one is dreaming, or that life really is a dream?

Dreaming provides a springboard for those who question whether our own reality may be an illusion. The ability of the brain to trick its host into believing a neuronally generated world is the "real world" moves the debate from statistically unlikely to a common, even nightly event.

Skeptics who fervently argue that the world is not simulated must concede that the mind making that claim is not itself a reliable mechanism for an analysis attempting to differentiate reality from illusion by nature of its own inability to distinguish between reality states.

This also silences those who claim a simulated reality requires far fetched scientific technology, since the only apparatus needed to construct a simulated reality is a human brain. And since human brains currently exist and consistently mimic our reality this also eliminates Occam's Razor as a valid defense for "realists" who dismiss the simulation hypothesis.

As mentioned earlier, the dream argument eliminates Occam's Razor as a valid defense against our own reality being simulated. Occam's Razor generally states that "all things being equal, the simpler explanation is preferable." Although this is not a scientific law, many skeptics defer to Occam's Razor as a means of avoiding the simulation hypothesis.

However, since we regularly create simulated realities in the form of dreams that fool those dreaming, the simple explanation could be that we're always being tricked by our brain or an outside mechanism. The existence of dreams must be accounted for when examining the equality requirement of Occam's Razor.

For example, in a world in which we never dreamt and were never fooled into believing a simulated reality was the "real world" then Occam's Razor could be a valid defense, since all things being equal it could be logical to assume we're not in a simulated reality.

Unfortunately, in a world in which we dream nightly and are incapable of distinguishing reality from simulation Occam's Razor would not tell us that the simple explanation is that we can trust our brains to inexplicably differentiate the two when we "wake up".

• Evil genius
• Lucid dreaming
• Maya (illusion)
• The Matrix - a fictional example of dream argument
• Reality in Buddhism - Reality and dreams in Dzogchen
• Simulation hypothesis
• Simulated reality
• Social simulation
• Voltaire
• Waking Life