It was in 1686, in what has since been given the title of the Discourse on Metaphysics, that Leibniz wrote the first systematic exposition of his philosophy. The central conception of the Discourse is the conception of individual created substance. Each complete individual in the world is active, but entirely self-contained. In it are to be found traces of all its past activities, and the ground of its present and future activities. Though all created substances are completely independent of one another, yet their activities are in thoroughgoing correspondence, and between them they make up a universe which is a perfect harmony.