This is the most up-to-date and comprehensive interpretation of the philosophy of Gottfried Wilhelm Leibniz (1646-1716). Among its other virtues, it makes considerable use of unpublished manuscript sources. The book seeks to demonstrate the systematic unity of Leibniz's thought, in which theodicy, ethics, metaphysics and natural philosophy cohere. The key, underlying idea of the system is the conception of nature as an order designed by God to maximize the opportunities for the exercise of reason.