On Keynes’s probability and uncertainty

Abstract There has been great confusion on Keynes’s notions of probability and uncertainty in recent years. Keynes believes that probability (his logical probability) is the guide of life, i.e., it is having some reasons to believe and to act. So probability is a theory of reasonable partial belief and a logic of non-demonstrative reasoning, upon which decision and action in conditions of limited knowledge, are based. (His) probability is, for Keynes, a positive, constructive, and forward-looking element of life. Limited knowledge is not a bar to decision and action. According to him, (his) probability helps us in almost all ordinary situations of life. This is the main reason why Keynes believes that ignorance and uncertainty are the two most difficult issues to tackle in life and in economics in particular. Both are related to a lack of limited knowledge. Ignorance is a lack of known reasons; Keynes writes we do not know. Keynes’s uncertainty is a much more intriguing concept than mere ignorance. Uncertainty is due to various reasons, one of them is the intrinsic incommensurability of probabilities. So, uncertainty is related to Keynes’s philosophy of measurement, a philosophy that also pervades his complex economics.