This comprehensive interpretation of the philosophy of Gottfried Wilhelm Leibniz (1646-1716) reveals the systematic unity of Leibniz's thought, in which theodicy, ethics, metaphysics, and natural philosophy contribute to a single vision of the best of all possible worlds. The key idea underlying the system is the conception of nature as a harmonious order designed by God to maximize opportunities for the exercise of reason. From this emerges an ethical ideal in which the virtue and happiness of human beings are promoted through the gradual extension of intellectual enlightenment. Professor Rutherford argues that Leibniz's metaphysical theories are best understood as attempts to model the complex order and harmony of nature, thereby providing support for the claims of his theodicy. Through detailed textual analyses involving many unpublished manuscript sources, the author demonstrates the essential unity of Leibniz's philosophy and its neglected moral underpinnings. This major contribution to Leibniz scholarship will prove invaluable to historians of philosophy, theology, and science.