On Formally Undecidable Propositions of Principia Mathematica and Related Systems (thing)

Or, in the original German, ``Über formal unentscheidbare Sätze der Principia Mathematica und verwandter Systeme''. The theorem proved therein is also known as Gödel's Incompleteness Theorem.

The proof of incompleteness assumes, of course, that said formal system is consistent.

Yes, outside links are naughty, but see http://www.ddc.net/ygg/etext/godel/index.htm