Skepticism
Philosophical skepticism is the challenge that we know far less than we suppose — perhaps nothing about the external world — and this area surveys its ancient and modern forms, the arguments that drive it, and the strategies philosophers have used to answer it.
Definition
Philosophical skepticism is the view, or the argumentative challenge, that we lack knowledge or justified belief in some broad domain — paradigmatically the external world — because we cannot rule out the possibility that we are radically deceived.
Scope
This area covers skepticism as a philosophical problem rather than ordinary doubt: the ancient Pyrrhonian project of suspending judgement, Descartes's dreaming and evil-demon arguments for external-world skepticism, the modern brain-in-a-vat version, and the closure-based skeletal argument that organises contemporary discussion. It includes the major responses — Moorean, contextualist, relevant-alternatives, and the rejection of closure. Detailed modal analyses of knowledge are treated in their own area.
Sub-topics
Core questions
- Can we know anything about the external world if we cannot rule out being dreaming or deceived?
- What is the difference between ancient Pyrrhonian and modern Cartesian skepticism?
- Is knowledge closed under known entailment, and what follows for skepticism?
- Which response to skepticism, if any, succeeds without begging the question?
Key theories
- Pyrrhonian skepticism
- The ancient sceptic, following Sextus Empiricus, sets opposing appearances against one another to induce suspension of judgement (epoché) and thereby tranquillity, rather than asserting that knowledge is impossible.
- Cartesian external-world skepticism
- Descartes uses the dreaming and evil-demon hypotheses to argue that, since one cannot rule out such global deception, one's beliefs about the external world are not certain knowledge.
- Closure-based skeptical argument
- The modern argument holds that I do not know I am not a brain in a vat, that knowing ordinary things entails knowing this, and so by closure that I do not know ordinary things either.
History
Skepticism originates with the ancient Pyrrhonists and Academics, whose techniques for suspending judgement Sextus Empiricus systematised. Descartes revived radical doubt in 1641 as a method for securing a foundation, generating the modern problem of the external world. Twentieth-century work, from Moore's commonsense proof to Stroud's diagnosis and the contextualist and relevant-alternatives theories, reframed skepticism around the closure principle.
Debates
- Does refuting skepticism require answering it on its own terms?
- Some, like Moore, reverse the argument and treat ordinary knowledge as more certain than skeptical premises, while others, following Stroud, argue that such responses fail to engage the skeptic's question and that a satisfying answer may be unattainable.
Key figures
- Sextus Empiricus
- René Descartes
- G. E. Moore
- Barry Stroud
Related topics
Seminal works
- sextus-outlines
- descartes-meditations
- stroud1984
Frequently asked questions
- Does skepticism claim that we know nothing?
- Some forms make that strong claim, but much philosophical skepticism is a challenge rather than a doctrine: it argues that our ordinary claims to knowledge cannot be defended against certain hypotheses, leaving it to others to show where the argument goes wrong. Ancient Pyrrhonism aimed only at suspending judgement.
- What is the brain-in-a-vat scenario?
- It is the modern counterpart of Descartes's evil demon: the hypothesis that you are merely a brain kept alive in a vat and fed experiences by a computer. Because such a brain's experiences would be indistinguishable from yours, the scenario is used to argue that you cannot rule it out and so cannot know you are not in it.