Scepticism and the external world: QCE Philosophy and Reason
“Explain and evaluate scepticism about the external world, including Descartes's dream and demon arguments and proposed responses”
Scepticism challenges our knowledge; global scepticism doubts all external-world knowledge by arguing our experience would be the same whether or not the world is as we believe. Descartes's method of doubt escalates from the dream argument (no certain mark divides waking from dreaming) to the evil-demon argument (a deceiver could falsify almost everything), aiming to find an indubitable foundation. He finds it in the cogito, then tries to rebuild knowledge via a non-deceiving God, a move criticised as the Cartesian circle. The modern brain-in-a-vat version updates the demon. Responses include Moore's appeal to what we know best, contextualism about shifting standards, the relevant-alternatives view, and Putnam's semantic argument, none decisive but each clarifying how demanding the sceptic's standard is.
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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.
Set out the brain-in-a-vat argument as numbered premises, then probe it.
Step 1: State the argument in standard form
P1: If I know I have hands, then I know I am not a (handless) brain in a vat. P2: I do not know I am not a brain in a vat (the scenario is experientially indistinguishable). C: Therefore I do not know I have hands.
Step 2: Confirm the form is valid
The argument is a valid modus tollens on P1: from "if K(hands) then K(not-vat)" and "not K(not-vat)", it follows "not K(hands)". So any escape must reject a premise.
Step 3: Locate the premise each response attacks
Moore rejects the conclusion by insisting P2 is less certain than "here is a hand". The relevant-alternatives view rejects P1's closure step, holding we need only rule out relevant alternatives, not the vat. Contextualism says P2 is true only when the sceptic raises the standards.
Step 4: Evaluate one reply
Take the relevant-alternatives reply: in ordinary life the vat is not a relevant alternative, so knowing I have hands does not require ruling it out, blocking P1. Strength: it preserves everyday knowledge. Weakness: it must give a principled account of which alternatives count as relevant, or it looks ad hoc.
Each sceptical scenario (dreaming, the demon, the vat) works by being experientially indistinguishable from normal life: if you would have the very same experiences either way, the sceptic argues you cannot be justified in ruling the scenario out. Most responses either lower the standard knowledge requires or deny that ruling out such far-fetched alternatives is necessary.
- Confusing global with local scepticism
- Global scepticism doubts all external-world knowledge; local scepticism targets a specific domain.
- Thinking the cogito rebuilds everything easily
- The cogito secures only that I exist as a thinker; Descartes's route back to the external world via God is widely challenged as circular.
- Treating Moore's proof as obviously decisive
- It may beg the question against the sceptic, which is exactly the point in dispute.
- Assuming the brain-in-a-vat scenario is refuted by its strangeness
- Strangeness is not refutation; the sceptic only needs it to be a possibility you cannot rule out.
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 questions
Questions in the style of QCAA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Practice7 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.
Practice5 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.