 Gödel had already noted this difference in the introduction to his 1929 thesis. The context was the following: Gödel entertains there the possibility that his proof of the Completeness Theorem might be circular, since the law of excluded middle was used to prove it. This is because while the Completeness Theorem asserts ‘a kind of decidability,’ namely every quantificational formula is either provable or a counterexample to it can be given, ‘the principle of the excluded middle seems to express nothing other than the decidability of every problem’: