# Keynes’s Logical Theory of Probability Is Concerned with Rational Degrees of Belief Based on Cognition, Which Are Not Mysterious or Mystical, Rank Ordered or Qualitative: F P Ramsey Failed to Grasp Keynes’s Use of Non–Additivity in His Reviews of the a Treatise on Probability

24 Pages Posted: 15 Jul 2019

Date Written: July 14, 2019

### Abstract

Recent exchanges between two academics on the Internet ,both experts in conventional and unconventional approaches to decision theory, reveal an astounding lack of knowledge and understanding regarding the makeup of Keynes’s logical theory of probability as put forth by Keynes in 1921, approximately 100 years ago.

Given this lacuna concerning Keynes, even after nearly 100 years has passed by, there is thus a need to restate some basic fundamental facts about Keynes's approach that can only be attained by reading beyond Part I of the A Treatise on Probability or Chapter III of the A Treatise on Probability. Unfortunately, what academics mean when they say that they have read the A Treatise on Probability is that they have read Part I and/or Chapter III of A Treatise on Probability or Chapter III of the A Treatise on Probability. Parts II, III, IV and V are simply skipped or skimmed over, as was done by F P Ramsey in his reviews of 1922 and 1926.

Keynes’s logical theory of probability is based on the upper–lower probability approach of George Boole. Boole is the explicit founder of the logical theory of probability that Keynes built on when writing the TP. Keynes called his approach inexact measurement and approximation. Keynes’s interval valued theory was designed, like Boole’s, to explicitly deal with non measurability, non comparability and incommensurability. Qualitative probability, comparative probability, ordinal probability, or rank ordered probability, just like numerical probability, can’t deal with non measurability, non comparability and incommensurability. The failure to absorb this basic point lies at the heart of all Post Keynesian and Fundamentalist Keynesian interpretations that all ignore George Boole’s fundamental contributions to Keynes’s logical theory of probability.

**Keywords:** logical probability, true probability, mysterious or mystical logical probabilities, Shackle, Boole, interval valued probability

**JEL Classification:** B10, B12, B14, B16, B20, B22

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