QuestionsIs the world real?The dream and the evil demon: can I be sure the world exists?
Is the world real?No way to know

The dream and the evil demon: can I be sure the world exists?

The dream argument; the evil demon (malicious genius); hyperbolic doubt; Cartesian doubt about the external world

René Descartes 1596 · 1650Doubt everything to find what survives doubt.
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What it claims

I cannot be certain that the external world exists: the sky, the air, the earth, colours, shapes and sounds could be mere illusions.

Why

Nothing surely distinguishes waking from dreaming, and a cunning being could give me the same sensations with no outside world.

The same idea through history
Zhuangzi300 BCZhuang Zhou dreams he is a butterfly, then wonders whether he is a butterfly dreaming he is Zhuang Zhou. It is the oldest account of the doubt between waking and dreaming in our library. He draws a lesson in wisdom from it more than a theory of knowledge.
Plato369 BCIn the Theaetetus (158b-d), Socrates reports the question that, he says, people often ask when they hold that what appears is true: what proof is there, here and now, that we are not asleep? Theaetetus says he can see none. Plato does not draw scepticism about the world from it: the argument serves to discuss the thesis that "knowledge is perception".
René Descartes1641Descartes adds to the dream argument a much stronger hypothesis, that of an all-powerful deceiver, and makes it the first step of his method. He himself leaves it behind: the doubt leads him to the cogito ("I am, I exist"), then to God, then to the world.Hilary Putnam1981Putnam tells the modern version: a brain in a vat connected to a computer, in place of the evil demon. He wants to refute it.
In short

"The dream and the evil demon: can I be sure the world exists?" is an argument by René Descartes (Descartes, Meditations on First Philosophy, Meditation I, 1641, 1641) on the question "Is the world real?" (answer: no way to know). Its claim: I cannot be certain that the external world exists: the sky, the air, the earth, colours, shapes and sounds could be mere illusions. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P2, P4). AskPhilosophy presents 5 objections, 1 of them with a documented reply, and the original text behind each premise, checked word for word.

Descartes, Meditations on First Philosophy, Meditation I, 1641
Valid with an implicit premise

Right now, you are reading these lines. But you have probably dreamt that you were reading, and suspected nothing until you woke up. Descartes starts from there, at the beginning of his Meditations (1641). He wants to find one absolutely certain truth. To do so, he decides to reject as false anything he can doubt, even slightly. Two reasons to doubt the external world come out of this: the dream (nothing surely tells waking from sleep), then the hypothesis of a very powerful "evil demon" who would deceive him about everything. Careful: this is not Descartes's conclusion. For him this doubt is a tool, not a doctrine. He leaves it himself in the following Meditations: he discovers that he exists while thinking (the cogito), then believes he proves the existence of a God who does not deceive, and so of the world. Here we present the sceptical argument in itself, in its strongest form, as generations of sceptics have taken it up.

P1There is no certain sign by which waking can be told from sleep: everything I am living right now could be a dream.
P2 · implicit premiseIf I cannot surely tell waking from dreaming, then I am not certain that the particular things I perceive (this hand, this paper) exist as I see them. But the dream leaves the simplest things intact (eyes, a body, colours), which Descartes grants are real, like a painter who can only invent mixtures of existing elements.
P3An all-powerful and cunning being could have arranged that I feel everything I feel without any earth, sky or extended body existing: this is the "evil demon" hypothesis. He could have deceived me even about what is most general.
P4 · implicit premiseIf two hypotheses give me exactly the same sensations, and I cannot rule out the one where the external world does not exist, then I do not have certainty that the external world exists.
Therefore I cannot be certain that the external world exists: the sky, the air, the earth, colours, shapes and sounds could be mere illusions.

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 P2 and P4, one can accept that dreaming is possible and the demon imaginable, and still say: "I know perfectly well that the world exists, even if I cannot prove it." Nothing in P1 and P3 alone forbids that.
The argument is valid. P1 and P2 make us doubt particular things. P3 rules out falling back on general things as well. P4 says that a hypothesis we cannot exclude is enough to remove certainty. Each premise is weak on its own; together they give the conclusion. P2 and P4 were added out of charity: Descartes assumes them without writing them.
Analogy. A player is plugged into a perfect virtual reality: he sees his controller, his hands, his room. Nothing in what he sees tells him whether it is the game or real life. As long as he cannot step out of the headset to compare, he has no certainty.

Objections

5

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

René Descartes (Meditation VI) 1641 · attacks premise 1

Descartes himself answers his dream argument at the end of the Meditations. For him there is a clear difference between the two states: memory can connect what I live awake with the rest of my life, while it never connects my dreams with each other or with my life. A dream appears and vanishes without connection, like a ghost. He therefore judges the doubts of the preceding days "hyperbolical and ridiculous" (exaggerated), especially the one about sleep.

no reply
Descartes never answered this objection.
The debate goes on (our summary) The debate goes on (our summary). Descartes himself does not stop at this criterion: on the same page he writes that, if my senses, memory and understanding agree, it is because God is no deceiver that I am not deceived. The coherence criterion therefore does not suffice on its own, and that is the next objection. A sceptic may also point out that a dream can contain false memories; but no text in our library develops this point.
René Descartes (Meditations III to VI) 1641 · attacks the conclusion

Descartes does not stay in doubt. After the cogito (I am, I exist, even if a demon deceives me), he believes he proves that a perfect God exists, from the idea he has of him (see the linked argument). But a perfect God is no deceiver: if my senses, memory and understanding agree, I am not deceived in this. The evil demon was only a working hypothesis, set aside afterwards.

Descartes replies
see the argument
A
Antoine Arnauld 1641
Arnauld, in the Fourth Objections, sees a circle. We can be sure that God exists only by conceiving it very clearly and very distinctly. But what we conceive clearly is certain only if God exists. To prove God, we would therefore already have to trust our reason, which only God guarantees.
René Descartes 1641 · Reply to the Fourth Objections
Descartes denies that there is a circle. He distinguishes what we conceive clearly at this moment from what we remember having once conceived clearly. We are first assured that God exists by attending to the reasons that prove it. Afterwards it is enough to remember having clearly conceived something to be sure it is true, and this presupposes that God exists and does not deceive.
The debate goes on (our summary) The debate goes on (our summary). Many readers find that Descartes's reply does not settle the "Cartesian circle": the memory of an evident truth is worth something only if memory is reliable, which the sceptic also doubts. This is one of the most discussed disagreements about the Meditations.
Where the debate stands (our summary)Disputed point. Scholars do not agree on how to read Descartes or on whether his reply succeeds. The underlying disagreement: can one leave the doubt without presupposing what it calls into question?
George Edward Moore 1939 · attacks the conclusion

In "Proof of an External World", Moore claims he can prove that the external world exists. He holds up his two hands and says "here is one hand", then "and here is another". He judges this proof perfectly rigorous. For him, we are more certain that our hands exist than of the truth of any premise of the sceptic.

no reply
see the argument
No reply possible: objection raised in 1939, after the author's death (1650).
The debate goes on (our summary) The debate goes on (our summary). Sceptics reply that Moore does not show where his knowledge comes from: he rejects the conclusion instead of refuting a premise. See the linked argument for the detail.
Hilary Putnam 1981 · attacks premise 3

Putnam takes up the demon hypothesis in the form of a brain in a vat, then argues that it refutes itself. If we were brains in a vat, our words would not refer to real vats or real brains. The sentence "we are brains in a vat" would then be false. It is therefore false in every case, and we can know that we are not in this situation.

no reply
see the argument
No reply possible: objection raised in 1981, after the author's death (1650).
The debate goes on (our summary) The debate goes on (our summary). Several philosophers hold that Putnam's proof assumes what the sceptic doubts, and leaves the recent-envatment hypothesis untouched. The linked argument details this debate.
J
John L. Austin 1962 · attacks premise 1

Austin rejects the idea that dreaming is like being awake. On Adam Leite's reading, he holds that waking experience can be told from dreaming, relying on ordinary contingent facts about dreams, and that we can therefore know we are not dreaming. Saying that "no sign allows us to tell" is, for him, an exaggeration.

no reply
No reply possible: objection raised in 1962, after the author's death (1650).
The debate goes on (our summary) The debate goes on (our summary). This reading is disputed: according to M. G. F. Martin, Austin does not really aim to refute the sceptic, but to show that the assumptions the sceptic and his opponent share are incoherent. We have not read Austin's chapter itself: this objection is given at second hand.