Imagine that a mad scientist created a machine into which he could place a human brain. This machine, which we shall call a “brain vat”, would not only keep the brain alive and functioning, but it would allow the scientist to create virtual stimuli and feed them directly into the brain. The brain would register all of these stimuli in exactly the same manner as normal human sensory experiences, as these are already interpreted as electrical signals anyway. In this way, the scientist could create an entire fictitious world that, to the captive brain, would feel completely normal.
What if I told you that you, the person reading this post, were not actually a human being but instead merely a brain in a vat? You may attempt to prove me wrong, but you would find that quite difficult, and you would not be alone. This thought experiment has puzzled philosophers since it was first proposed in 1641 by René Descartes (Though Descartes’ experiment used an evil demon in place of a vat. The vat was proposed Gilbert Harman in 1973 to update the experiment to accommodate modern understandings of psychology and neuroscience). The idea of the brain in a vat (BIV) is that no brain could ever know whether it was in a skull or a vat, and could therefore never know whether everything it experiences is real or an illusion.
Descartes answered his own version of the experiment with his famous cogito, ergo sum (“I think, therefore I am”). I do not have the time or space to fully explain the Cogito in this post, but I highly encourage everyone to read about it. Essentially Descartes argues that either the world is real and he is experiencing it, or he is being deceived. Even if he is being deceived, he still exists in order to be deceived. Therefore, the fact that he can question his existence is sufficient to prove that he exists.
Importantly, the Cogito does not prove that he is not being deceived (or, to use the BIV terms, that he is not a brain in a vat). What Descartes instead proves is that he is something, not necessarily a human or even necessarily a brain, but something. Based on the Cogito, a BIV can know that it exists, but it cannot know anything else about itself or the world.
This thought experiment has implications for ethics (if you are a brain in a vat and nothing else is real, there is nothing wrong with doing terrible things to others), epistemology (the study of knowledge and what it means/why it matters), our understandings of what it means to be human, and many other philosophical disciplines. Many scientists have also written about and studied this thought experiment for a variety of reasons and in a variety of contexts.
On a lighter note, if this all sounded very familiar, it may be because this thought experiment is the basic plot of The Matrix (though the matrix also includes some elements of the experiment from last week’s post, The Allegory of the Cave). On an even lighter note, here are some funny cartoons about this thought experiment. Have a great weekend, even if it’s all an illusion!