The economic theories of John Maynard Keynes descend in large measure from his views on the nature of uncertainty, probability, measurability, and the theory of knowledge as presented in the Treatise on Probability. This single, uninterrupted influence is readily recognizable throughout his theoretical economic writings as well as his policy pronouncements and excursions into areas other than the economic (e.g., population control and social policy matters). The main ideas derived from the Treatise which are consistent throughout Keynes’s economic writings, in particular those discussed here, viz., A Tract on Monetary Reform, the Treatise on Money, the General Theory of Employment, Interest, and Money, and the 1937 Quarterly Journal of Economics article, ‘The General Theory of Employment’, are the notions of the non-measurability of beliefs and the non-seriable quality of many empiricallyobserved events, and the uncertainty of individual perceptions and apprehensions of the economic, political, and social environments. In other words, Keynesan economics is predicated on an understanding of systemic and epistemic uncertainty (with the emphasis on the epistemic) as fundaments of a theory of choice in an open system. Keynes was consistent in his conviction that individual, subjective choice among alternative courses of action could not be understood within the context of a deterministic environment. Nondeterminism, a realization of the importance of the individual, and a general rejection of a condition of stasis as a description of the economic milieu are persistent and crucial notions appearing throughout his work. Taken in concert with an advocacy of subjectivism, the result is a theory in many ways consistent with that of Austrianism.