What if I were just a brain in a vat?

The brain in a vat (sceptical hypothesis, and Putnam's reply)

Hilary Putnam 1926 · 2016The same mental state can be realised by very different brains: the mind is a function, not a stuff.

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What it claims

I do not know that I have hands, nor, more generally, that the external world exists as I see it.

Why

A brain in a vat wired to a computer would have the same inner life as mine: no experience tells the two situations apart.

The same idea through history

René Descartes1641In the first Meditation, Descartes imagines an all-powerful evil demon who deceives him about everything, and considers himself as having no hands. The doubt already covers the external world and the body.

Hilary Putnam1981Putnam tells the modern version: a mad scientist, a vat of nutrients, a computer sending signals to the nerves. He uses it to raise the old problem of scepticism about the external world, and then to fight it. The film The Matrix (1999) made this image famous.

In short

"What if I were just a brain in a vat?" is an argument by Hilary Putnam (Reason, Truth and History, 1981, chapter 1, 1981) on the question "Is the world real?" (answer: the world is real). Its claim: I do not know that I have hands, nor, more generally, that the external world exists as I see it. According to AskPhilosophy's logic check (Z3 solver), the conclusion does follow from its 1 premises. AskPhilosophy presents 3 objections, 1 of them with a documented reply, and the original text behind each premise, checked word for word.

Reason, Truth and History, 1981, chapter 1

Valid

You look at your hands. Are you sure they exist? Imagine that your brain has been removed from your body, placed in a vat, and connected to a computer that sends you exactly the sensations of a normal life. Nothing in what you live would let you tell. This is the modern sceptic's argument, which Putnam sets out at the start of his 1981 book. Careful: Putnam does not defend it, he wants to refute it. The argument below is therefore the sceptic's, in its strongest form. The main objection is Putnam's own: "we are brains in a vat" is a sentence that refutes itself, because the words of a brain in a vat would not refer to real vats (this is called semantic externalism: the meaning of our words depends on what they are connected to in the world).

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 P3, one could accept the vat story and still say: I know I have hands, because my eyes and hands show me, even if I cannot prove that the vat does not exist.

The argument is valid. P1 and P2 give: I do not know that I am not a brain in a vat. P3 says that knowing I have hands requires knowing this. So I do not know that I have hands. The two hidden principles, P2 and P3, carry almost the whole debate: P3 is challenged by Moore and by contextualism, and P2 by Putnam.

Analogy. A player in a perfect virtual reality game sees the controller, hands and room. Nothing tells him whether it is the game or real life. So he cannot be sure, from the inside, that his hands are real.

Objections

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

Hilary Putnam 1981 · attacks premise 2

The sceptic is wrong: I can know that I am not a brain in a vat, because the hypothesis refutes itself. If all thinking beings had always been brains in vats, their word "vat" would refer to images or computer signals, not to real vats, since it would never have been causally connected to real vats. Their sentence "we are brains in a vat" would therefore mean something other than what our sentence means. If we were brains in vats, that sentence would be false. So it is false in every case.

Putnam replies

Michael Kinghan, then Gary Ebbs (as summarised in the Stanford Encyclopedia) 1986

Putnam's proof assumes what the sceptic doubts. To say that the word "vat" of a brain in a vat does not refer to real vats, one must accept a theory of reference drawn from our ordinary beliefs. But if I suspend my ordinary beliefs to the point of supposing I may be a brain in a vat, it is unclear why I would be entitled to rely on that theory.

Hilary Putnam 1981 · Putnam on his own method (1981 text, written before the objection)

Putnam describes his own procedure: it asks what is reasonably possible, assuming certain very general premises. It is neither "empirical" nor quite "a priori": it depends on assumptions that might be called empirical (for example that the mind has no access to external things apart from the senses). He thus admits that his proof is fallible.

See the rest of the debate (2)

David Christensen 1993

Even if Putnam's proof works, it leaves another hypothesis untouched: my brain has just been put in a vat without my knowing. My words would then have their ordinary meaning and the sentence "I am a brain in a vat" would be true. The problem posed by this hypothesis is as strong and as plausible as Putnam's.

The debate goes on (our summary) The debate goes on. Christensen holds that this reply does not change the substance: it uses ordinary beliefs, exactly what the sceptic doubts. Putnam's defenders answer that his argument never aimed to prove we cannot be deceived, only that the hypothesis of the eternal vat is incoherent. Neither side has won.

Joseph Tymoczko, Hilary Putnam (1998), then Thorpe and Wright (as summarised in the Stanford Encyclopedia) 1989

To rule out the recent-envatment hypothesis, we may rely not on semantics but on our ordinary beliefs about the current state of medical technology: nobody can transplant a brain into a vat and connect it to a computer able to simulate an entire life.

George Edward Moore 1939 · attacks the conclusion

In "Proof of an External World", Moore claims he can prove that the external world exists: he raises his hands and says "here is one hand, and here is another". He is, he says in substance, more certain that he has hands than of any of the sceptic's premises. If the sceptic's argument is valid and concludes that I do not know I have hands, then one of its premises must be false, even if we do not know which.

no reply

Putnam never answered this objection.

The debate goes on (our summary) The debate goes on (our summary). Sceptics reply that Moore does not show where his knowledge comes from and merely refuses the conclusion. This argument is treated separately on the site ("Here is one hand").

Keith DeRose 1995 · attacks premise 3

The word "know" does not have the same requirements in every situation. When the sceptic talks about vats, he raises the standard so high that his conclusion ("you do not know you have hands") is true in that context. But in everyday life, where the standard is lower, I know perfectly well that I have hands. The sceptic's argument is therefore true at its own standard, and our ordinary knowledge stays intact.

no reply

Putnam never answered this objection.

The debate goes on (our summary) The debate goes on (our summary). Critics hold that this solution does not really answer the sceptic: it grants him his point in his own context, and the question "do we really know?" remains. Neither side has won.

What today's voices say

Defend it

David ChalmersTakes the simulation hypothesis as the modern form of Descartes' sceptical scenarios, but argues that being in a simulation would not make the world unreal. Answering Putnam's brain-in-a-vat argument, he says someone in a simulation who says "I'm in a simulation" would speak truly.