brains in a vat

"I want now to ask a question which will seem very silly and obvious .., but which will take us to real philosophical depths rather quickly. [..]
The argument I am going to present is an unusual one, and it took me several years to convince myself that it is really right. [..]
It first occurred to me when I was thinking about a theorem in modern logic, the Skolem-Löwenheim Theorem ..."

Hilary Putnam about brains in a vat and related thoughts about the mind-body problem.

kind of sets