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INTRODUCTION
[To Gödel's Theorem]
by
R. B. BRAITHWAITE
Every system of arithmetic contains arithmetical propositions, by which is meant propositions concerned solely with relations between whole numbers, which can neither be proved nor be disproved within the system. This epoch-making discovery by Kurt Gödel, a young Austrian mathematician, was announced by him to the Vienna Academy of Sciences in 1930 and was published, with a detailed proof, in a paper in the Monatshefte für Mathematik und Physik Volume 38 pp. 173-198 (Leipzig: 1931). This paper, entitled "Über formal unentseheidbare Sätze der Principia Mathematica und verwandter Systeme I" ("On formally undecidable propositions of Principia Mathematica and related systems I"), is translated in this book. Gödel intended to write a second part to the paper but this has never been published.
Gödel's Theorem, as a simple corollary of Proposition VI is frequently called, proves that there are arithmetical propositions which are undecidable (i.e. neither provable nor disprovable) within their arithmetical system, and the proof proceeds by actually specifying such a proposition, namely the proposition g expressed by the formula to which "17 Gen r" refers [188]. g is an arithmetical proposition; but the proposition that g is undecidable within the system is not an arithmetical proposition, since it is concerned with provability within an arithmetical system, and this is a metaarithmetical and not an arithmetical notion. Gödel's Theorem is thus a result which belongs not to mathematics but to metamathematics, the name given by Hilbert to the study of rigorous proof in mathematics and symbolic logic.