This study of the philosophical roots of economics examines the impact on the 18th century economic thought of the rivalry between two opposing philosophical outlooks - rationalism and anti-rationalism. It argues that the economic thought of this period was a synthesis of these two outlooks. The book deals with an intellectual conflict which remains relevant today. Neoclassical economics is frequently critisized on account of those of its assumptions which are rationalist in character, in particular its assumptions of optimization, rationality and equilibrium. This book digs into the intellectual archaeology of the continuing controversy over neoclassical economics, and offers some new perspectives.