[wsy]

They know that they know nothing, so the question of proofs is somewhat moot.

[klez]

Not really. That was just Socrates being Socrates and being all humble about his knowledge. Apart from him I don't remember any othe philosopher adopting that stance.

[tommorris]

> I don't remember any othe philosopher adopting that stance.

Peter Unger's book "Ignorance" (Oxford University Press, 1975) is probably the best modern defence of extreme skepticism.

Also, Unger is pretty fun to read.