This book introduces the economic theories that have placed uncertainty at the core of economic discourse. Covering the works of John Maynard Keynes and Frank Knight, as well as those of John von Neumann, Oskar Morgenstern, and Leonard Savage, it reconstructs the complex history of how economists have analyzed uncertainty, as distinct from risk, in the twentieth century. The text delves into the question of the extent to which uncertainty is a measurable entity, focusing on the perspective of those authors who have attempted to demonstrate its irreducibility to risk, with particular regard to Daniel Ellsberg and the ensuing current decision theory. The book will appeal to scholars of the history of economic thought, as well as students of advanced microeconomics, decision theory, game theory, and history of economic thought.