Scepticism

An Enquiry Concerning Human Understanding

David Hume

Causation is a habit of expectation rather than an observed necessity, and induction has no non-circular justification.

Assumes some acquaintance with what Locke claimed about ideas — Hume is dismantling it

Brains in a Vat

Hilary Putnam

Argues that the sceptical hypothesis that we are brains in a vat is self-undermining, because a genuine brain in a vat could never have the word 'vat' refer to real vats in the first place.