Cartesian and External-World Skepticism

External-world skepticism is the worry that, because radical deception is conceivable, none of our beliefs about the world beyond our own minds can amount to knowledge — a worry Descartes dramatised with the dreaming and evil-demon hypotheses and modern philosophers with the brain in a vat.

Definition

Cartesian or external-world skepticism is the position that, since we cannot rule out hypotheses of radical deception under which all our sensory experience would be exactly as it is, we lack knowledge of the external world.

Scope

This topic covers the modern problem of the external world: Descartes's method of doubt, the dreaming argument, and the evil-demon hypothesis, together with their contemporary descendant, the brain-in-a-vat scenario. It examines how these hypotheses are constructed to be empirically indistinguishable from ordinary experience and how they generate the skeptical conclusion. The closure principle that formalises the argument and the responses to it are developed in companion topics.

Core questions

• Can we tell, from the inside, whether we are dreaming or being deceived?
• Why does the conceivability of an evil demon threaten knowledge of the world?
• How does the brain-in-a-vat scenario update Descartes's argument?
• Does the very meaning of our words undermine the brain-in-a-vat hypothesis?

Key theories

The dreaming argument

Descartes notes that dreams can be subjectively indistinguishable from waking experience, so any belief based on the senses might be a dream, casting doubt on all such beliefs.

The evil-demon hypothesis

Descartes supposes a powerful malicious deceiver who makes all his experiences misleading; since he cannot rule this out, he cannot be certain of anything based on the senses, even mathematics.

Putnam's semantic reply to brain-in-a-vat

Putnam argues that, given a causal theory of reference, the words of an envatted brain could not refer to real brains or vats, so the sentence 'I am a brain in a vat' is, if uttered by such a brain, false — a transcendental argument against the hypothesis.

History

Descartes set out the dreaming and evil-demon arguments in the First Meditation of 1641, using them to clear the ground for a certain foundation in the cogito. Stripped of Descartes's theological reconstruction, the arguments became the standing problem of the external world. In 1981 Putnam restated the challenge as the brain-in-a-vat hypothesis and offered an influential semantic argument that it is self-refuting.

Debates

Whether the brain-in-a-vat hypothesis is self-refuting

Putnam contends that semantic externalism makes the hypothesis impossible to state truly from within, but critics argue that the argument at most shows we cannot truly say we are vat-brains, not that we can know we are not, leaving the epistemic challenge intact.

Key figures

• René Descartes
• Barry Stroud
• Hilary Putnam

Seminal works

• descartes-meditations
• putnam1981

Frequently asked questions

Why is the evil demon such a powerful skeptical device?

Because it is designed to be undetectable: a sufficiently powerful deceiver could make every experience and reasoning seem exactly as it does now. If you cannot find any feature of your experience that rules the demon out, you cannot use experience to justify the claim that the world is as it appears.

Is the brain-in-a-vat scenario just science fiction?

Its scientific plausibility is beside the point. What matters epistemically is that the scenario seems coherent and would be subjectively indistinguishable from ordinary life, so the question is whether you can know it is false, not whether it is technologically feasible.