Abstract

The purpose of this paper is to re-assess the role of uncertainty in Keynes’s macroeconom-ics by exploring the concepts of probability, rational belief and rational action, as they ap-pear in the Treatise on Probability. Three main claims are then made. First, there exist sub-stantial links between the conception of human rationality in the Treatise on Probability and that in the General Theory. Secondly, Keynes attributes considerable importance to the ration-al elements involved in the process of investment decision and suggests that they normally lead to macroeconomic stability. Instability is thus explained by a specific institutional shock to an economy, which is already prone to crises due to the imperfect working of certain key markets. From this perspective, the post-Keynesian literature is misleading in blaming uncer-tainty for capitalist instability and unemployment. Thirdly, following Keynes’s hints, I sug-gest that an institutionalist research programme would provide a firm basis to model invest-ment behaviour under uncertainty in a more realistic way than usually done.