Keynes and probability: An assessment

Most interpreters agree that Keynes had a wide-ranging, complex, 'vision of the world', which underlies his theoretical contributions. Whenever this is forgotten, as happens in the so-called neoclassical synthesis, not only the original Keynesian spirit goes lost but also, and especially, we lose substantive bricks for our theoretical constructions. The paper considers an important instance of this general rule; namely Keynes's views on the logic of probability, meant as the field concerning human behaviour in an uncertain world (hence connected to, but distinct from, the pure theory of probability, meant as a field of mathematics). The paper begins by recalling the main aspects of the classical and frequentist approaches to probability and the main criticisms they received, pertaining among other things to the limits of their applicability. We then consider Keynes's own views, stressing three aspects: the definition of probability as pertaining to the field of logic, the notion of uncertainty and of the 'weight of the argument', the 'theory of groups'. We then discuss the subjective approach of de Finetti, Ramsey and Savage, and contrast it with Keynes's own views. Finally, we consider the implications of our analysis for the interpretation of Keynes's General Theory, and of his attitude towards econometrics.