It will be useful to review the state of business cycle theory at the publication of The General Theory. In September, the League of Nations commissioned Gottfried Haberler to do a study of business cycles (or “trade cycles”). The study was to cover both theory and statistical testing of theory. The statistical work was interrupted by the onset of World War II and apparently never completed. The theoretical volume was divided into two sections: Part I, a survey of then-current theories of the business cycle, and Part II, an attempt by Haberler to synthesize these theories into a theory of the cycle. This volume, with the title, Prosperity and Depression, was first published as a League of Nations document in 1936. In one of those coincidences dear to the heart of a historian, the preface to a later, public edition, tells us that the manuscript of the 1936 volume “was substantially completed” in December 1935. Keynes’s Preface to The General Theory is dated 13 December 1935. Thus, we have, precisely dated, the last word on business cycle theory at the time of the completion of The General Theory.