Are you a brain in a vat?

The setup

A mad scientist removes a person's brain and places it in a vat of nutrients that keep it alive. Its nerve endings are connected to a supercomputer, which sends it exactly the electrical signals a normal brain receives. The brain experiences walking through a park, eating a sandwich, reading a page about brains in vats. None of it is real.

The disturbing part is not that this could happen to someone else. It is that, if it were happening to you, you could not tell. Every experience you have had, including this one, would be just as it is now. Any test you devised to check would itself be part of the simulation.

The brain in a vat is a modern, scientific version of René Descartes's Descartes' evil demon. It powers the central argument for skepticism: if you cannot rule out that you are a brain in a vat, how can you know anything about the world outside your mind?

Why it matters

The scenario forces the question of what knowledge requires. If knowing that you have hands requires ruling out every scenario in which you would have the same experiences without hands, then you know almost nothing.

It also anticipates real technology. Virtual reality and brain-computer interfaces raise questions about whether experiences in simulated environments are less real, and whether a life lived entirely in one could be a good life.

Key terms

• SkepticismIn philosophy, the view that we know much less than we think, or nothing at all, or the practice of doubting claims to knowledge.
• Semantic externalismThe view that the meanings of our words, and the contents of our thoughts, depend partly on our environment, not only on what is in our heads.
• Closure principleThe principle that if you know something, and know that it implies something else, you know that other thing too. Sceptical arguments often rely on it.
• Simulation hypothesisThe hypothesis that our world, including ourselves, is a computer simulation run by some more advanced civilisation.

The positions

Arguments and objections

Who argued what

• We cannot know we are not Barry Stroud, Peter Unger
• Putnam: the hypothesis refutes itself Hilary Putnam
• We know through the best explanation Jonathan Vogel, Bertrand Russell

Where you stand

Variations

Go deeper

Chalmers: you would not be deceived about much

David Chalmers argues that even if you are a brain in a vat, most of your beliefs might be true. Your belief that you have hands would be about the digital objects you interact with, which really exist as computational structures. On this view, the vat scenario is not a skeptical scenario at all, but a discovery about what the world is made of. See the simulation argument.

Further reading and sources

• EncyclopediaStanford Encyclopedia of Philosophy: Skepticism and Content Externalism
• WikipediaBrain in a vat
• SurveyPhilPapers Survey, 2020 · question: Response to external-world skepticism; about 1,785 professional philosophers; accept or lean toward
• BookReason, Truth and History, Hilary Putnam
• BookReality+, David Chalmers

Open questions

• What would you do differently if you knew you were a brain in a vat?
• If a simulated life is indistinguishable from a real one, is it any less real?
• Can you mean 'vat' if you have never touched one?