"Suppose
evil scientists removed your brain from your body while you slept, and set it
up in a life-support system in a vat. Suppose they then set out to trick you
into believing that you were not just a brain in a vat, but still up and about,
engaging in a normally embodied round of activities in the real world. This
old saw, the brain in the vat, is a favorite thought experiment in the toolkit
of many philosophers. It is a modern-day version of Descartes's (1641) evil
demon, an imagined illusionist bent on tricking Descartes about absolutely everything,
including his own existence."
Daniel Dennett: Consciousness Explained, London, 1993 edition, Penguin, p. 3.