Reinhold Baer

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The main result of Gödel's paper ... can be formulated in a pointed and, hence not fully correct way as follows: if A is a countable, consistent logical system, for example arithmetic, then there is a proposition in A which is not decidable in A (but may be decidable in extended systems). Hurrah, logicians have also discovered diagonalisation!
Letter to Zermelo in 1931