The ‘Brain in a Vat’ thought experiment

7 March 2018

Arguably the most famous thought experiment on knowledge and reality, this exercise asks you to imagine that a mad scientist (or another entity) has taken your brain from your body and placed it in a vat of some life-sustaining fluid. Electrodes have been connected to your brain, and these are also linked to a computer that generates images and sensations. So the brain can still think and act as if it were still in a skull, but all the information it processes comes from the computer, which has the ability to simulate your everyday experience. Assuming this was scientifically possible, how would you ever know whether the world around you was real, or just a simulation generated by a computer? From the point of view of that brain, it would be impossible to tell.

The experiment – also called ‘the brain in a jar’ – has been discussed both amongst philosophers and more widely, in film and literature. The Matrix is a well-known example. In philosophy, the exercise reminds us of Descartes’ Meditations, where he questions whether his sensations were really his own, or just a dream, or an illusion caused by an “evil demon”. (Note that this is only a stage in Descartes’ line of interrogation and not the conclusive one).


  • What view of reality and knowledge can this thought experiment be used to support?
  • If someone told you today that your brain was removed at birth, put in a vat and attached to electrodes, and that nothing you’ve seen, heard, or done was real – it was all simulated, what argument could you invoke against this?
  • What if we don’t even know if our brains are real? What if our minds are stimulated directed by an evil demon? What could we say against that?
  • Now, think about the version of scepticism presented in The Matrix. Is it more similar to –
    • Plato’s cave,
    • Descartes’ dream or demon hypothesis, or
    • The brain in a vat scenario?