---
title: Scepticism and Its Responses
section-id: epistemology
sort: 140
author: Prof. James Okafor
created: 2026-01-24 09:00
modified: 2026-03-01 11:00
language: en
description: Sceptical arguments about the external world and the major philosophical responses.
---

# Scepticism and Its Responses

Scepticism is the philosophical position that knowledge—or justified belief—in some domain is impossible, or at least very difficult to achieve. Epistemological scepticism about the external world holds that we cannot know whether our perceptual experiences accurately represent a mind-independent reality.

## The Dream Argument

Descartes, in the *Meditations on First Philosophy* (1641), poses the dream argument. When dreaming, we have experiences indistinguishable from waking experience—or at least, we cannot tell from inside the dream that we are dreaming. If we cannot distinguish dreaming from waking, how do we know we are not dreaming now? And if we might be dreaming now, how do we know any of our perceptual beliefs are true?

The argument does not establish that we are dreaming. It establishes that we cannot rule out that we are, and this seems to be enough to undercut our claim to knowledge. Knowledge requires ruling out relevant alternatives, and the dreaming hypothesis is a relevant alternative that we cannot rule out.

## The Evil Demon

Descartes extended the sceptical challenge with the hypothesis of an evil demon (*malin génie*): an omnipotent deceiver who causes all our experiences while we have no bodies, no external world, and no reliable cognitive faculties. Even our seemingly secure mathematical beliefs might be the product of systematic deception.

The evil demon sets the sceptical challenge at its most extreme. It is the ancestor of the modern **brain-in-a-vat** hypothesis: suppose you are a disembodied brain kept alive in a vat and fed experiences by a supercomputer. All your experiences would be just as they are. How do you know you are not a brain in a vat?

## Responses to Scepticism

### Descartes’ Own Response: The Cogito and God
Descartes used the cogito (‘I think, therefore I am’) to establish at least one certainty. From there he argued for the existence of a non-deceiving God who guarantees the reliability of clear and distinct perceptions. This response has been widely criticised for circular reasoning: Descartes uses the reliability of clear and distinct perceptions to establish God’s existence, then uses God’s existence to validate that reliability (the ‘Cartesian circle’).

### Common Sense Philosophy: Reid and Moore
Thomas Reid argued that certain beliefs—in the external world, in other minds, in the continuity of the self—are **basic beliefs** that do not require justification from more fundamental premises. They are the starting points of rational inquiry, not conclusions to be derived. Attacking them with philosophical arguments is like trying to prove that you exist: the attempt itself presupposes what is in question.

G.E. Moore offered a famous response to scepticism. ‘Here is a hand,’ he said, holding up his hand. ‘Here is another.’ He claimed to know these propositions with certainty. Since the existence of external objects follows, scepticism is refuted.

The sceptic will reply: Moore may be certain, but his certainty does not address the question of whether he is right. Certainty and correctness can come apart.

### Contextualism
Knowledge attributions, on contextualist views, are sensitive to context. In ordinary contexts, we know a great deal; the standards are not very demanding. In philosophical discussions, the standards rise—we raise alternatives (evil demons, brains in vats) that we do not normally consider relevant. The sceptic’s error is to import the elevated philosophical standards into ordinary contexts.

### Closure Denial
Some epistemologists deny the **closure principle**: the principle that if you know P, and you know that P entails Q, then you know Q. Sceptical arguments rely on closure. If you know you have a hand, and you know that having a hand entails you are not a brain in a vat, then you know you are not a brain in a vat. But the sceptic argues you do not know the latter, so you do not know the former.

Fred Dretske and Robert Nozick have argued that knowledge does not close under known entailment. Your belief that you have a hand is sensitive to the facts about your hand—if you did not have a hand, you would not believe you do. But your belief that you are not a brain in a vat is not sensitive in the same way: if you were a brain in a vat, you would still believe you were not. So the two beliefs have different epistemic statuses, and closure fails.

### Wittgenstein on Hinge Propositions
In *On Certainty*, Wittgenstein argued that certain propositions function as **hinges**—not as things we know but as the fixed points around which inquiry pivots. ‘I have a body,’ ‘the world existed before I was born,’ ‘other people have minds’—these are not conclusions we could sensibly doubt because doubting them would undermine the entire practice of reasoning that gives doubt its meaning.

## Summary

- Sceptical arguments (dream argument, evil demon, brain in a vat) challenge whether we can know anything about the external world
- Descartes’ own response is widely considered unsuccessful due to the Cartesian circle
- Common sense philosophers (Reid, Moore) treat basic beliefs as not requiring justification
- Contextualists relativise knowledge standards to context; closure deniers reject the closure principle
- Wittgenstein treats certain propositions as hinges rather than objects of knowledge
