---
title: Scepticism and Its Responses
sort: 140
section-id: epistemology
description: Cartesian scepticism, the brain-in-a-vat scenario, contextualism, and relevant alternatives theories.
language: en
---

# Scepticism and Its Responses

Scepticism is the philosophical position that knowledge — or at least, some significant domain of knowledge — is impossible. It has been a central problem in epistemology from antiquity to the present, partly because it is remarkably difficult to refute and partly because attempting to refute it has driven some of philosophy's most creative work.

## Ancient Scepticism

The ancient Greek sceptics — Pyrrho of Elis, and later the Academic sceptics including Arcesilaus and Carneades — argued that for any claim, there is an equally strong case for its denial, leaving the rational response one of *epoché*: suspension of judgment ^[Sextus Empiricus, *Outlines of Pyrrhonism*, I.8-12, c.200 CE]. Suspension of judgment, they argued, brings *ataraxia* — tranquillity, freedom from the anxiety that dogmatic belief produces.

Ancient scepticism was primarily a practical philosophy — a way of living without commitment to metaphysical positions. Modern scepticism takes a different form: it is primarily an epistemological challenge, asking whether knowledge is possible given the limitations of our access to reality.

## Cartesian Scepticism

Descartes' sceptical scenarios, introduced in the *Meditations* as methodological tools, have become the canonical statements of modern epistemological scepticism. The *dreaming argument*: I cannot rule out that I am dreaming right now, and if I might be dreaming, I cannot be certain that anything I currently believe is true ^[Descartes, *Meditations*, AT VII:19].

The *evil demon hypothesis* is more radical: suppose there is an infinitely powerful deceiving demon who ensures that all my beliefs — including the deliverances of reason and mathematics — are false. I cannot disprove this. Therefore, I cannot be certain of anything.

The sceptical strategy exploits what has been called *epistemic closure*: if I know that P entails Q, and I know P, then I know Q. Equivalently: if I don't know Q, and P entails Q, then I don't know P ^[Nozick, R., *Philosophical Explanations*, p.204]. Sceptics argue: if I knew that I have hands, I would know that I am not a brain in a vat; I do not know that I am not a brain in a vat; therefore, I do not know that I have hands.

## The Brain-in-a-Vat Scenario

Hilary Putnam updated Descartes' evil demon into the brain-in-a-vat scenario ^[Putnam, H., *Reason, Truth and History*, 1981, ch.1]. Suppose my brain has been removed from my body, placed in a vat, and is being fed electrical signals by a supercomputer that simulates a complete reality. All my experiences are exactly as they would be in normal embodied life.

Putnam argued — controversially — that this scenario is incoherent. A brain in a vat lacks the causal connections to the external world that are necessary for its terms to refer to external objects. "Water," as a brain-in-a-vat thinks it, does not refer to H₂O — it refers, at most, to the computer simulation. So a brain-in-a-vat thinking "I am not a brain in a vat" is producing a true sentence — because its words do not refer to the things that would make it false. This semantic argument against scepticism has been widely discussed and contested.

## Responses to Scepticism

**Moorean Responses.** G.E. Moore's response to scepticism was blunt: we know more certainly that we have hands than we know any philosophical premise used in the argument for scepticism ^[Moore, G.E., "Proof of an External World", *Proceedings of the British Academy* 25, 1939]. The *modus ponens* can be run in either direction: from the premises to the sceptical conclusion, or from the falsity of the sceptical conclusion to the falsity of one of the premises. Moore insisted the latter is more reasonable.

Wittgenstein developed a related response: certain propositions — "There are physical objects," "The world has existed for many years" — function as *hinges* that cannot be doubted within any practice of inquiry, because doubting them would not be coherent inquiry but something else entirely ^[Wittgenstein, L., *On Certainty*, §341, 1951].

**Relevant Alternatives.** Fred Dretske proposed that knowledge requires ruling out only *relevant* alternatives — possibilities that are live given one's actual situation ^[Dretske, F., "Epistemic Operators", *Journal of Philosophy* 67, 1970]. The possibility that I am a brain in a vat is not a relevant alternative in ordinary contexts; I do not need to rule it out to know that I have hands. Scepticism artificially expands the class of alternatives that must be eliminated.

**Contextualism.** David Lewis and Stewart Cohen developed contextualist responses: the standards for knowledge vary with context ^[Lewis, D., "Elusive Knowledge", *Australasian Journal of Philosophy* 74, 1996; Cohen, S., "How to be a Fallibilist", *Philosophical Perspectives* 2, 1988]. In ordinary contexts, we correctly say we know many things. In sceptical philosophical discussions, where very high standards are in play, those knowledge attributions are false — but this does not undermine ordinary attributions, which operate at a lower standard. Scepticism is a local phenomenon of artificially elevated epistemic standards.

**Externalist Responses.** If knowledge requires reliably produced beliefs (as reliabilism holds), then the sceptical demon scenario involves beliefs that are not reliably produced and therefore do not constitute knowledge. But Descartes' scenario is just that — a scenario where knowledge fails. This does not show that we actually lack knowledge in the real world.

## Closure Denial

Nozick's tracking theory denied epistemic closure, which blocks the sceptical argument at its source ^[Nozick, R., *Philosophical Explanations*, pp.204-211]. On his account, I know I have hands because if I didn't have hands I wouldn't believe I did (the sensitivity condition). But I do not know I am not a brain in a vat — because if I were a brain in a vat, I would still believe I wasn't (the sensitivity condition fails). Yet the failure to know the second proposition does not undermine knowledge of the first, because the inference from "I have hands" to "I am not a brain in a vat" is not knowledge-preserving on Nozick's account.

This is elegant, but the denial of closure is philosophically costly and has not won widespread acceptance.

## The Significance of Scepticism

Scepticism matters not primarily because it is a live hypothesis that reflective people adopt, but because engaging with it illuminates the structure of our knowledge and the character of epistemic justification. The sceptical challenge to close our eyes and demonstrate that we know anything about the external world has driven epistemologists to produce their most careful accounts of justification, reliability, and the conditions for knowledge. Scepticism is philosophy's sharpening stone.
