Consider this: right now, you are not where you think you are. In fact, you happen to be the subject of a science experiment being conducted by an evil genius.
Your brain has been expertly removed from your body and is being kept alive in a vat of nutrients that sits on a laboratory bench.
The nerve endings of your brain are connected to a supercomputer that feeds you all the sensations of everyday life. This is why you think you’re living a completely normal life.
Do you still exist? Are you still even “you”? And is the world as you know it a figment of your imagination or an illusion constructed by this evil scientist?
Sounds like a nightmare scenario. But can you say with absolute certainty that it’s not true?
Could you prove to someone that you aren’t actually a brain in a vat?
Deceiving demons
The philosopher Hilary Putnam proposed this famous version of the brain-in-a-vat thought experiment in his 1981 book, Reason, Truth and History, but it is essentially an updated version of the French philosopher René Descartes’ notion of the Evil Genius from his 1641 Meditations on First Philosophy.
While such thought experiments might seem glib – and perhaps a little unsettling – they serve a useful purpose. They are used by philosophers to investigate what beliefs we can hold to be true and, as a result, what kind of knowledge we can have about ourselves and the world around us.
Descartes thought the best way to do this was to start by doubting everything, and building our knowledge from there. Using this sceptical approach, he claimed that only a core of absolute certainty will serve as a reliable foundation for knowledge. He said:
If you would be a real seeker after truth, it is necessary that at least once in your life you doubt, as far as possible, all things.
Descartes believed everyone could engage in this kind of philosophical thinking. In one of his works, he describes a scene where he is sitting in front of a log fire in his wooden cabin, smoking his pipe.
He asks if he can trust that the pipe is in his hands or his slippers are on his feet. He notes that his senses have deceived him in the past, and anything that has been deceptive once previously cannot be relied upon. Therefore he cannot be sure that his senses are reliable.