The three great historical philosophers most often associated with rationalism - Descartes, Spinoza and Leibniz - opened up ingeneous and breathtaking vistas upon the world. Yet their works are so difficult that readers often find themselves stymied. Understanding Rationalism offers a guide for anyone approaching these thinkers for the first time. It unlocks their intricate metaphysical systems, which are by turns surprising, compelling and sometimes bizarre. It also lays out their controversial stanceson moral, political and religious problems. The study is framed by an opening discussion of the broad themes and attitudes common to these three philosophers and a closing analysis of the legacy they left for the rest of philosophy.