"Brain in a Vat" was first proposed by Hilary Putnam in 1981 in his book *Reason, Truth, and History*.
The basic theory is as follows:
A person (let's assume it's you) has been operated on by an evil scientist. Their brain is removed from their body and placed in a vat of nutrient solution that keeps it alive.
The brain's nerve endings are connected to a computer, which feeds the brain information according to a program, causing it to have the illusion that everything is completely normal.
For this person, it seems that people, objects, and the sky still exist. Their own movements and physical sensations can all be fed to them as input.
This brain can also have memories implanted or extracted. It can even be fed code to make it 'feel' as if it is currently reading an interesting and absurd piece of text.
