Coel has put up a post on one of philosophy’s favourite topics: Brain in a vat (BIV). I’ll try to get to other sources, such as the Massimo Pigliucci post Coel refers to, but for now I’ll respond to Coel’s post only.
So, “[the BIV problem] supposes that we are a brain kept alive in a vat, being fed with a stream of inputs generated by an Evil Genius. Everything that we experience as sense data is not real, but is artificially simulated and fed to us.”