The Discourse on Metaphysics of 1686 is generally regarded as the first complete presentation of Leibniz's mature metaphysics. In this chapter, we trace the development of that philosophy from Leibniz's youth, through his years in Paris, to his time in Hanover. Because the metaphysics of the 1680s has lately received so much attention and because the importance of the earlier philosophy has generally not been recognized, we concentrate on Leibniz's thought prior to 1680. In section I, we present the intellectual context in which his youthful metaphysics is most easily understood and summarize both his original metaphysical principles and his first conception of substance. We claim that these metaphysical principles, all of which concern substance, form the bedrock of Leibniz's philosophy for years to come. In section 2, we unearth an inconsistency that Leibniz discovered between his first account of substance and the principles, and trace the steps he took in revising the former. In section 3, we argue that this concept of substance, combined with certain theological commitments, led Leibniz to develop most of the central doctrines of his mature thought. For example, we claim that by April, 1676, Leibniz has arrived at his doctrine of preestablished harmony. In section 4, we discuss the relationship between his concept containment theory of truth, which grew out of intensive work on logic in 1679, and his theory of substance. We finally give a brief summary of the central doctrines of the metaphysics of the Discourse.