The idea that the solitary thinker, using pure reason, can unlock the deepest mysteries of reality is certainly an exciting one. The rationalist dream began with Parmenides of Elea and became the epistemological foundation for the greatest metaphysical systems ever constructed. This thesis traces the complete historical development of this neglected epistemology and inaugurates a radical new appraisal of its method. The new clarity provides fresh insights into traditional puzzles like the Cartesian Circle and whether Plato had a secret esoteric doctrine. A new positive re-evaluation of a doctrine thought to have been refuted by Kant and the positivists will provide interest for students, philosophers and those interested in the history of ideas.