René Descartes was a seventeenth century French philosopher famous for his method of systematic doubt. Descartes proposed a number of thought experiments which he thought showed how even our most basic, commonsense beliefs could be subject to sceptical doubts. One of the most extreme examples of such thought experiments was his so-called ‘evil demon’, sometimes called the Cartesian demon. The idea of this thought experiment is simple. Suppose that some sort of supernatural being existed whose sole purpose was to mislead and deceive you. Any and all of your thoughts, experiences, and perceptions could be manipulated by this demon so as to cause you to believe things which are in fact false. For example, you may believe that you are reading this paragraph right now, but the evil demon could be deceiving you by implanting the illusion in your mind of reading this paragraph, when in fact this paragraph never existed at all, and you are in fact doing something else entirely.
A modern version of the evil demon argument is the so-called ‘brain in a vat’ hypothesis. The idea is that any of us could in fact not exist in the ‘real world’ we believe ourselves to inhabit, but instead be merely a brain kept in a vat my a mad scientist, all of our perceptions shaped by the patterns of electrical activity fed to our brain by the scientist in order to deceive us. This idea of our entire world being a deceptive virtual reality is famously explored in the movie The Matrix.
The purpose of these sceptical thought experiments is not to argue that there actually is an evil demon or that we really are brains in vats. Rather, their goal is to expose even our most basic beliefs to the possibility of radical doubt. Since there seems to be no way we could rule out that we are being deceived by an evil demon or a mad scientist (after all, how could we possibly tell if we were?), the argument concludes that we may not in fact be justified in any of our beliefs that depend upon our not being so deceived. So, for example, it might seem obvious that we can see our hand extended in front of us, but since we have no ability to rule out the possibility that an evil demon is deceiving us to believe this, we cannot be justified in this belief, and therefore cannot accord it with the status of knowledge.
Few philosophers believe that Descartes’ evil demon or similar arguments really do undermine our ability to possess knowledge. Exactly how to best respond to such cases, however, is widely disputed, and has been subject to extensive discussion in the philosophical literature. Such thought experiments are, however, useful for helping us think more clearly and rigorously about the foundations of knowledge and proper responses to doubt.
Important arguments from Descartes’ Meditations: an accessible outline of Descartes’ core skeptical argument
Rene Descartes: Sparknotes summary of Descartes’ arguments in Meditations
Descartes Epistemology: detailed discussion from Standard Encyclopedia of Philosophy
The argument from deception: brief overview of the evil demon argument from TheoryOfKnowledge.info