Cartesian skeptics invite us to explain how knowledge of (or justified belief about) the external world is possible given the challenge that we cannot know (or justifiably believe) the denials of skeptical hypotheses, such as that one is dreaming or a brain-in-a-vat. The problem has its source in Rene Descartes’ Meditations on First Philosophy, and in particular, the First Meditation.
For example, Descartes considers the hypothesis that there is a powerful evil demon who renders his beliefs about the world false, while making it seem to him just as if they are true. The challenge Descartes raises is this: how can we know that the evil demon hypothesis is false? External world skepticism is view that that knowledge (or justified belief) about the external world is impossible. An external world skeptic is a Cartesian skeptic if they appeal to skeptical hypotheses in order to show that we cannot know (or justifiably believe) anything about the external world. The Cartesian skeptical argument is often presented as follows: (1) If you know that an external world proposition P is true, then you know that the skeptical hypothesis SH is false. But (2) you don’t know that SH is false. Therefore, (3) you do not know that P.
Responses to the Cartesian skeptical argument can be divided into those which maintain that premise 1 is defective and those which maintain that premise 2 is defective. Mooreans rejects premise 2, arguing that we can know the denials of skeptical hypotheses by way of competent deduction from our ordinary external world knowledge. Different theorists have tried to support the Moorean view in different ways (e.g., dogmatist, reliabilist, knowledge-first, and disjunctivists). Relative Alternatives Theorists argue that skeptical hypotheses are not relevant alternatives to our ordinary knowledge. Many relevant alternative’s theorists reject the closure-principle and thereby premise 2. Contextualists argue that the truth-conditions of our knowledge-ascriptions are sensitive to context, allowing that, in ordinary contexts, ascriptions of ‘S knows that P’ can be true even though, in skeptical contexts, ascriptions of ‘S knows that P’ are false.
Some philosophers argue that there are pragmatic reasons to maintain our ordinary beliefs, even if they are in some way epistemically defective according to Cartesian skeptics. Veridicalists have argued that some skeptical hypotheses, like the BIV or simulation hypothesis, are compatible with the truth of our ordinary beliefs.