This is the most up-to-date and comprehensive interpretation of the philosophy of Gottfried Wilhelm Leibniz (1646-1716). Amongst its other virtues, it makes considerable use of unpublished manuscript sources.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 maximise the opportunities for the exercise of reason. From this idea emerges the view that this world is the best of all possible worlds, and an ethical ideal in which the well-being of human beings is promoted through the gradual extension of intellectual enlightenment.