Radical skepticism (or radical scepticism in British English) is the philosophical position that knowledge is most likely impossible. Radical skeptics hold that doubt exists as to the veracity of every belief and that certainty is therefore never justified. To determine the extent to which it is possible to respond to radical skeptical challenges is the task of epistemology or "the theory of knowledge".
Several Ancient Greek philosophers, including Plato, Cratylus, Pyrrho, Arcesilaus, Carneades, Aenesidemus, Agrippa the Skeptic, and Sextus Empiricus have been viewed as having expounded radically skeptic positions. Three of the Hellenistic philosophies held radically skeptic views: Pyrrhonism, Academic Skepticism, and Cyrenaicism. In ancient Greek the radical skeptical view was termed acatalepsy, denoting the ungraspablity of knowledge.
In modern philosophy, two representatives of radical skepticism are Michel de Montaigne (most famously known for his skeptical remark, Que sçay-je ?, 'What do I know?' in Middle French; modern French Que sais-je ?) and David Hume (particularly as set out in A Treatise of Human Nature, Book 1: "Of the Understanding").
As radical skepticism can be used as an objection for most or all beliefs, many philosophers have attempted to refute it. For example, Bertrand Russell wrote “Skepticism, while logically impeccable, is psychologically impossible, and there is an element of frivolous insincerity in any philosophy which pretends to accept it.”
- Feyerabend, Paul (1999). For and against Method. Chicago: University of Chicago Press. p. 395. ISBN 0-226-46775-9.
- Dancy, Jonathan (1993). A Companion to Epistemology. Oxford: Blackwell. p. 89. ISBN 0-631-19258-1.
- Russell, Bertrand (1948). Human knowledge, its scope and limits. New York: Simon and Schuster. pp. 9. OCLC 373835.