Post your favorite proof

[brewer]

Knock-down argument/proof against universal skepticism:

U: Universal skepticism: For any assertion P, we don't know if P is true or false.

Proof: The assertion U: "For any assertion P, we don't know if P is true or false" is either true or false. If it's true, we know at least that one assertion is true (U itself) therefore it's false. QED.

You're very yeasty, and yet un-risen.

I'll call you Hoastie.