QuestionsIs the world real?The butterfly dream: how do I know I am not dreaming?
Is the world real?No way to know

The butterfly dream: how do I know I am not dreaming?

Zhuangzi's dream; the dream argument; Zhuang Zhou dreams he is a butterfly

Zhuangzi c. 369 BC · c. 286 BCOur views of the world shift like dreams: the sage welcomes them all and follows the transformation of things.
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What it claims

I cannot be certain of the reality of what I perceive: I must hold my certainties loosely.

Why

In a dream everything feels real, and nothing in the present moment proves that I am awake rather than dreaming.

The same idea through history
Zhuangzi300 BCIn the last paragraph of chapter 2, Zhuang Zhou tells how he dreamt he was a butterfly, then asks who is dreaming whom. The text reaches us in the version edited by Guo Xiang (d. 312).
René Descartes1641Descartes takes up the dream question in the first Meditation (I cannot surely tell waking from sleep), then thinks he settles it in the sixth.
In short

"The butterfly dream: how do I know I am not dreaming?" is an argument by Zhuangzi (Zhuangzi, chapter 2 (4th c. BC), 300 BC) on the question "Is the world real?" (answer: no way to know). Its claim: I cannot be certain of the reality of what I perceive: I must hold my certainties loosely. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P3). AskPhilosophy presents 3 objections, 0 of them with a documented reply, and the original text behind each premise, checked word for word.

Zhuangzi, chapter 2 (4th c. BC)
Valid with an implicit premise

One morning you wake up relieved: "It was only a dream." But during that dream you were sure it was real. What guarantees that you are not dreaming right now? More than two thousand years ago, Zhuangzi told how he dreamt he was a butterfly, and on waking no longer knew whether he was a man who had dreamt of being a butterfly or a butterfly dreaming of being a man. Note that Zhuangzi does not give a formal proof. The P1 to C form below is our reconstruction of the dream argument as the philosophical tradition developed it from this story. Zhuangzi himself draws something else from it, as the first objection shows.

P1When I dream, I feel awake: things seem as real and as present as in waking life.
P2Nothing in my experience at the moment lets me be sure that I am awake rather than dreaming: I can ask whether I am a man who dreamt he was a butterfly, or the reverse.
P3 · implicit premiseIf I cannot be sure that I am awake, then I cannot be sure that what I perceive right now is real.
Therefore I cannot be certain of the reality of what I perceive: I must hold my certainties loosely.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P3, one can accept P1 and P2 and still believe the table in front of one is real: one merely admits one cannot demonstrate it. Many philosophers say that probability, or a good reason to believe, is enough for knowing.
The argument is valid: P1 says the dream resembles waking life, P2 that nothing lets me tell them apart surely at the present moment, and P3 draws the conclusion that the reality of my perceptions is not certain. The weak point is P2: is there really no criterion? This is where Descartes and Austin attack. A second point is P3, which can be disputed, since not being a hundred percent sure is not the same as having no reason to believe.
Analogy. Imagine a virtual reality film so perfect that, headset on, you see no difference from real life. As long as you wear the headset, you cannot know whether you are in the film. Dreaming is the headset that nature puts on us every night.

Objections

3

Each objection is a conversation: click to see who replies and where the debate stands.

Zhuangzi (the end of the passage) 300 BC · attacks the conclusion

The text does not end on "I cannot know". Zhuangzi adds that there is nevertheless a difference between Zhuang Zhou and the butterfly, and that this passage from one to the other is called "the transformation of things" (wuhua). He does not say that reality cannot be found. He wants to show that our distinctions (me and butterfly, waking and dreaming, life and death) are not fixed: things change into one another. The sage draws from this not the anguish of doubt but the suppleness of following the transformation. The butterfly serves to loosen our attachment to our points of view, not to prove that we know nothing.

no reply
Zhuangzi never answered this objection.
The debate goes on (our summary) This reading is common, but its exact sense is debated: the sentence is short, and Giles's English translation (1889) renders it as "Metempsychosis", which is probably a misreading of the Chinese word, better rendered "transformation of things". The dream argument as we reconstruct it is therefore a Western reading of the story, not what Zhuangzi puts first in it. The two readings can meet: if our categories are relative, we have good reason to distrust our certainties.
René Descartes 1641 · attacks premise 2

Descartes himself had raised the dream doubt in the first Meditation. In the sixth, he thinks he has lifted it: waking differs from dreaming because memory can connect waking events to the whole course of life, while it can never connect dreams with each other and with our life. If someone appeared and vanished at once, without my seeing whence he came or whither he went, I would take him for a phantom. When I can connect what I perceive with the rest of my life, I am sure I am awake. So there is a criterion: coherence.

no reply
Zhuangzi never answered this objection.
The debate goes on (our summary) Many readers find the criterion weak: a long, coherent dream is not impossible, and the memory of my past can itself be part of the dream. Descartes also adds that this certainty rests on God not being a deceiver, which leads back to the "Cartesian circle" (see Agrippa's trilemma). The criterion gives a very good clue, not an absolute proof.
J
John L. Austin 1962 · attacks premise 2

Austin, a philosopher of ordinary language, denies that dreaming and being awake are two experiences impossible to tell apart. We know very well, in general, how to tell the difference: that is what gives meaning to the phrase "it was only a dream". And one cannot doubt everything at once: a doubt makes sense only if there is a specific reason to doubt in the situation. The sceptic, in his view, abuses our words ("dream", "real") by stripping them of the context that gives them meaning.

no reply
Zhuangzi never answered this objection.
The debate goes on (our summary) Sceptics reply that the question is not whether we usually tell dreams apart, but whether we can be certain we are not mistaken this time. Our summary of Austin is a general one: we could not re-read the book, so the page reference is still to be checked.