Interval Probabilities, and Not Ordinal Probabilities, are the Foundation of J M Keynes's Approach to Probability

Abstract

J M Keynes’s wide ranging discussions of interval estimates and their application in chapter III of the A Treatise on Probability (TP,1921) was mistaken by Frank Ramsey to be a discussion of ordinal estimation in 1922 and 1926. Ramsey completely misunderstood how Keynes’s logical theory of probability was operationalized. This catastrophic, intellectual blunder was then passed on from Frank Ramsey by way of Gay Meeks and Robert Skidelsky to Rod O’Donnell, Bradley Bateman, Anna Carabelli, Jochen Runde, and a host of others. Philosophers were not immune. Henry E Kyburg and Isaac Levi, for instance, also concluded that Keynes had to have been working with ordinal probabilities because they limited their reading of the TP to Part I. This paper pinpoints where in chapter III this devastating blunder by Ramsey occurred. Ramsey mistook Keynes‘s discussions of interval probabilities for discussions of ordinal probabilities. Ramsey made one of the greatest intellectual blunders in the history of science and can no longer be considered a major contributor to work in decision theory and probability.