5.4 Scepticism, Externalism and the Ethics of Belief

What this lecture covers

The lecture closes the unit on knowledge and scepticism by testing whether externalism about knowledge can survive sceptical pressure, and whether a clever linguistic move by Hilary Putnam can dissolve the brain-in-a-vat worry. It follows on from earlier lectures on the analysis of knowledge and externalist accounts of justification.

By the end, you should be able to explain why externalism does not fully defeat the sceptic, state Putnam's semantic argument against vat scepticism and its main weakness, and describe why Hume's appeal to the ethics of belief offers a more modest way of living with scepticism than trying to refute it outright.

Key ideas

• Externalism about knowledge: knowing depends on reliable causal connections between the world and your faculties, not on being able to justify your beliefs from the inside; this lets dogs, cats and unreflective people count as knowers.
• The God's-eye problem: even if externalism is true, you cannot know from your own internal perspective that your faculties are in fact working reliably, so sceptical doubt can still be raised about your own case.
• Putnam's semantic argument: if you are a brain in a vat, the word 'hand' in your mouth would refer to hand-images, not real hands, so your statement 'there's a hand' would still come out true.
• Objection to Putnam: the argument only works if your word 'hand' was learned while envatted; someone kidnapped and envatted after learning language from real hands would still mean 'real hand', so the sceptical worry returns.
• Vertical vs horizontal scepticism: vertical scepticism questions the inference from perceptions to an external world; horizontal scepticism (the problem of induction) questions whether the future will resemble the past, and it survives even if you accept living in a vat or matrix.
• The ethics of belief: rather than demanding certainty before believing anything, Hume's approach asks what we should believe given that suspending all belief is psychologically impossible.
• Living with less than certainty: because animal nature makes us believe things without full internal justification, externalism fits naturally with accepting fallible standards instead of holding out for an unreachable ideal.

Before you watch

• Be familiar with the analysis of knowledge and the externalism/internalism debate covered in earlier lectures in this course.
• Know the basic brain-in-a-vat and matrix scenarios as standard sceptical thought experiments.
• Recall Hume's treatment of induction, since the lecture contrasts it with vertical scepticism.

Check your understanding

1. Why does externalism about knowledge fail to fully silence the sceptic, even if it is true?
2. Explain Putnam's argument for why 'there's a hand' comes out true even for a brain in a vat, and state the strongest objection to it.
3. What is the difference between vertical and horizontal scepticism, and why does the lecture treat horizontal scepticism as potentially more worrying?
4. How does appealing to the ethics of belief change the terms of the debate with the sceptic?