Can we know that the external world exists, or could all our experience be a systematic illusion?
explain and evaluate scepticism about the external world, including Descartes's dream and demon arguments and proposed responses
A focused QCE Unit 4 answer on philosophical scepticism. Covers global versus local scepticism, Descartes's dream and evil-demon arguments, the cogito and his attempted escape, the modern brain-in-a-vat version, and responses including Moore's proof and contextualism.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
QCAA wants you to confront the sceptical challenge that we cannot know the most basic things, such as that there is an external world. You need to distinguish kinds of scepticism, reconstruct Descartes's sceptical arguments (the dream and the evil demon) and his attempted escape, state the modern brain-in-a-vat version, and evaluate the main responses. This caps the theory-of-knowledge strand.
The answer
What scepticism claims
Philosophical scepticism challenges our claims to knowledge. Local scepticism doubts a specific area (for example knowledge of other minds or the future); global scepticism doubts whether we can know anything about the external world at all. The sceptic typically argues that for all I can tell, my experience would be exactly the same even if the world were not as I believe, so my beliefs are not justified in the way knowledge requires.
Descartes's method of doubt
Rene Descartes, in the Meditations (1641), set out to find an indubitable foundation for knowledge by doubting everything that could possibly be false. His sceptical arguments escalate:
- The dream argument: there is no certain mark distinguishing waking from dreaming, so for all I know I am dreaming now, and my belief that I am sitting here could be false.
- The evil demon argument: suppose a powerful deceiver makes me have all my experiences while nothing external is real, even manipulating my reasoning about simple matters. Then almost any belief could be false.
These arguments aim to show that ordinary perceptual beliefs, and even some mathematical ones, are not absolutely certain.
The cogito and the attempted escape
Descartes finds one thing the demon cannot fake: while I am thinking, I cannot doubt that I am thinking, so "I think, therefore I am" (the cogito) is indubitable. From this foundation he tries to rebuild knowledge: he argues that a non-deceiving God exists and would not let his clear and distinct perceptions be systematically false, so the external world can be trusted. Critics object that this reasoning is circular (the Cartesian circle): he uses clear and distinct ideas to prove God, then uses God to validate clear and distinct ideas.
The brain in a vat
The modern version: you might be a disembodied brain in a vat, fed exactly the experiences you now have by a computer. Since the experience would be indistinguishable from real life, how can you know you are not such a brain? If you cannot rule it out, the sceptic says, you cannot know you have hands or that the external world exists. This is a vivid form of global external-world scepticism.
Responses
- G. E. Moore's "proof": Moore held up his hands and argued "here is one hand, here is another, so external things exist," claiming the certainty of his hands outweighs any sceptical premise. Critics say this begs the question; defenders say it rightly trusts what we know best over abstract doubts.
- Contextualism (Keith DeRose and others): whether a knowledge claim is true depends on context; in ordinary contexts "I know I have hands" is true, while in the sceptic's high-stakes context the standards rise so it is false. Knowledge claims are not false everywhere, only under raised standards.
- Relevant alternatives / closure-denial: we need only rule out relevant alternatives, not far-fetched ones like the demon, so ordinary knowledge survives.
- Putnam's semantic argument: a brain in a vat could not even mean "brain" or "vat" in the way required, so the sceptical hypothesis may be self-undermining.
No response is universally agreed to defeat scepticism, but each shows how high a standard the sceptic is demanding and whether knowledge really requires it.
Try this
Q1. Distinguish global from local scepticism. [2 marks]
- Cue. Global doubts all external-world knowledge; local doubts a specific domain such as the future or other minds.
Q2. Reconstruct Descartes's evil-demon argument. [4 marks]
- Cue. A powerful deceiver could give me all my experiences while nothing external is real, so almost any belief could be false.
Q3. Explain the contextualist response to scepticism. [3 marks]
- Cue. Knowledge standards shift with context; ordinary claims are true at everyday standards, false only when the sceptic raises them.
Exam-style practice questions
Practice questions written in the style of QCAA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
QCAA 20227 marksReconstruct Descartes's sceptical arguments about the external world and evaluate his attempt to escape them via the cogito and a non-deceiving God.Show worked answer →
A 7 mark response reconstructs the arguments, the escape, and the standard objection.
The arguments. The dream argument: there is no certain mark dividing waking from dreaming, so my belief that I am sitting here could be false. The evil-demon argument escalates: a powerful deceiver could give me all my experiences while nothing external is real, and even tamper with simple reasoning, so almost any belief could be false.
The escape. Descartes finds one indubitable point: while thinking, I cannot doubt that I think, so "I think, therefore I am" (the cogito) survives the demon. From this he argues a non-deceiving God exists and would not let his clear and distinct perceptions be systematically false, restoring trust in the external world.
Evaluation. The cogito secures only the thinker's existence, not the external world. The route back via God faces the Cartesian circle: clear and distinct ideas are used to prove God, then God is used to validate clear and distinct ideas. So the escape is widely judged to fail.
Markers reward accurate reconstruction of both arguments, the cogito, and the Cartesian-circle objection.
QCAA 20235 marksExplain the brain-in-a-vat sceptical hypothesis and evaluate the contextualist response to it.Show worked answer →
A 5 mark response states the hypothesis and assesses contextualism.
The hypothesis. You might be a disembodied brain in a vat, fed exactly the experiences you now have by a computer. Because the experience would be indistinguishable from real life, you seemingly cannot rule it out, so (the sceptic argues) you cannot know you have hands or that an external world exists.
Contextualist response. Keith DeRose and others hold that the truth of a knowledge claim depends on context. In ordinary contexts the standards are low and "I know I have hands" is true; when the sceptic raises the stakes, the standards rise so the same sentence becomes false. Knowledge is not lost everywhere, only under raised standards.
Evaluation. Contextualism elegantly explains why sceptical arguments feel compelling yet everyday knowledge claims seem true, but critics object that it makes knowledge oddly unstable and does not show the sceptic's high standard is illegitimate.
Markers reward an accurate statement of the vat hypothesis (indistinguishability) and a critical account of contextualism.