Are you a brain in a vat?

Excerpted by Adam Elga, with minor changes, from Michael Huemer's "Epistemology: contemporary readings."

All of your brain’s information about the world comes from electrical impulses that your sense organs and nerves send to your brain. Now, imagine the following scenario. Scientists in a technologically advanced society have figured out how to keep a brain alive, floating in a vat. They have also developed technology for directly stimulating a brain electrically in order to produce a complete set of experiences as of normal life. So why should you believe that this scenario is not actually true, and that you are not, right now, a brain in a vat?

You shouldn’t. That’s because all the evidence you have for or against claims about the external world comes from your sensory experiences. But in the brain-in-a-vat scenario, you would be having the same sort of sensory experiences you are in fact having. So your actual sensory experiences are not evidence against the brain-in-a-vat scenario. So you have no evidence that you are not a brain in a vat. So you shouldn’t believe that you are not a brain in a vat.