What is a "brain in a vat"? Imagine that one day a mad scientist takes your brain out of your body and puts it in a vat filled with nutrient solution to keep it alive. This brain is connected to a supercomputer through electrodes, and the computer transmits the same electrical signals to the brain through these electrodes that we perceive in normal life. In other words, this brain will still "perceive" the world around it, and it may think that it is still living in a normal body and leading a normal life, even though it is actually just an isolated brain in a vat. --niji 6