Philosophical skepticism is a family of positions and methods in epistemology that challenge claims to knowledge or epistemic justification. Rather than merely questioning an isolated assertion, it examines whether the grounds ordinarily offered for belief can establish what they purport to establish. Some forms deny that particular kinds of knowledge are available; others recommend suspending judgment without declaring knowledge impossible. Its central questions concern the reliability of cognition, the adequacy of evidence, and the conditions under which beliefs count as knowledge. (plato.stanford.edu)
Scope and distinctions
Skepticism may be local, directed at a particular domain, or global, extending across knowledge more generally. A skeptic about the external world questions whether sensory experience establishes the existence or character of mind-independent objects. Other skeptical arguments concern memory, other minds, or predictions about unobserved events. Their targets need not include every proposition: skepticism about perception can coexist with acceptance of logical or mathematical knowledge. (plato.stanford.edu)
A further distinction separates denying knowledge from withholding assent. The assertion that nobody knows a proposition is itself a claim; suspending judgment avoids affirming either that proposition or its negation. Skepticism also differs from fallibilism, which allows that knowledge-giving evidence need not guarantee truth. Recognizing the possibility of error therefore does not, by itself, entail that knowledge is unavailable. (plato.stanford.edu)
Ancient traditions
The Greek term skepsis means investigation. In ancient Greece, skepticism developed principally through the Academic and Pyrrhonian traditions. Ancient skeptics treated inquiry not only as an intellectual activity but also as a way of living, with particular attention to belief, appearances, and suspension of judgment. (plato.stanford.edu)
Academic skepticism emerged within the Academy founded by Plato, especially under Arcesilaus and Carneades. Academic arguments challenged the Stoic claim that certain impressions could furnish an infallible criterion of truth. Carneades explored how persuasive impressions might guide action without providing the certainty demanded by that criterion; interpretations differ over whether such proposals expressed his own commitments or served dialectical purposes. (plato.stanford.edu)
Pyrrhonism, associated with Pyrrho and known chiefly through Sextus Empiricus, opposed arguments and appearances in ways intended to produce suspension of judgment, or epochē. Sextus connected this suspension with ataraxia, tranquility concerning matters of opinion. Pyrrhonists continued inquiry rather than asserting that truth could never be discovered. They also distinguished following everyday appearances, customs, and practical skills from endorsing theories about reality. (plato.stanford.edu)
Skeptical arguments
One major challenge concerns the regress of justification. When a belief is supported by another belief, the supporting belief may itself require justification. Repeated demands appear to lead to an infinite chain, circular support, or an unsupported stopping point. The skeptic questions whether any of these structures supplies adequate grounds for belief. (plato.stanford.edu)
Different theories respond by rejecting different assumptions. Foundationalism recognizes basic beliefs whose justification does not depend on inference from other beliefs. Coherentism emphasizes relations within a system of beliefs, while infinitism permits an unending structure of reasons. Their disagreements concern what makes support genuinely justificatory, rather than merely psychologically persuasive. (plato.stanford.edu)
Another argument employs skeptical hypotheses: possibilities apparently indistinguishable from ordinary experience but incompatible with familiar knowledge claims. Dreaming, systematic deception, and the brain-in-a-vat scenario are prominent examples. These thought experiments ask whether experience can discriminate between ordinary circumstances and radically misleading alternatives. They do not require that such alternatives be probable. (iep.utm.edu)
The epistemic closure principle helps formulate this challenge: roughly, knowledge is preserved through competent deduction from known premises. If knowing that one has hands would enable one to know that one is not a handless, deceived subject, inability to know the latter appears to threaten knowledge of the former. The debate concerns both closure and the alleged inability to exclude the skeptical alternative. (plato.stanford.edu)
Early modern developments
In Meditations on First Philosophy (1641), René Descartes used methodical doubt to seek secure foundations. Sensory error, dreaming, and an imagined powerful deceiver progressively widened the range of beliefs placed in doubt. Descartes was not advocating permanent skepticism: the act of doubting disclosed the doubter’s existence, and his subsequent arguments attempted to reconstruct knowledge. His method illustrates how skeptical reasoning can serve an anti-skeptical project. (plato.stanford.edu)
David Hume developed a different challenge through the problem of induction. Past observations do not deductively establish that unobserved cases will resemble observed ones. Appealing to induction’s previous success appears circular if the reliability of inductive reasoning is precisely what requires justification. Hume explained our expectations through custom or habit. Scholars disagree about the exact normative force of his argument, including whether it denies rational justification or chiefly challenges a particular account of it. (plato.stanford.edu)
Responses and continuing debates
G. E. Moore defended familiar commonsense propositions and famously presented his hands as proof that external objects exist. Moorean responses give ordinary knowledge claims greater credibility than premises used to undermine them. Critics question whether this answers the skeptic’s demand for justification; Moore distinguished proving an external world from disproving skepticism. (iep.utm.edu)
Epistemic contextualism instead holds that the standards governing knowledge attributions vary with conversational context. Ordinary claims may be true under everyday standards, while discussion of skeptical possibilities raises those standards. Ludwig Wittgenstein explored another approach in On Certainty: doubt and inquiry operate against background commitments that are not ordinarily tested like empirical hypotheses. These approaches dispute whether radical skepticism identifies a genuine limitation of knowledge, shifts the standards for its attribution, or detaches doubt from the practices that give it meaning. (iep.utm.edu)