  For Leibniz, the fundamental questions of metaphysics were reducible to questions of ontology: What is there? What are the most basic components of reality? What grounds what? In a certain sense, his answer remained constant throughout his life: everything is composed of or reducible to simple substances; everything is grounded in simple substances. While Leibniz appears to have given slightly different accounts of the precise nature of these simple substances over the course of his career, there are many features that remained constant in his mature philosophy: Leibniz always believed that a substance had a “complete individual concept” and that it was essentially an active unity endowed with perception and appetition.