 Whether passive substances truly exist or not, active agencies “are responsible for whatever activity is attributed to the former” (Shapere, 45). Leibniz’s principle (which has it ancestors principally in Stoicism) is this: “Whatever is fundamentally explanatory must be an active principle” (Shapere, 45). The purely passive cannot do anything unless it is acted upon. At times, Leibniz seems to think that only the truly active exists: “we can show from the inner truths of metaphysics that what is not active is nothing, for there is no such thing as a mere potentiality to act without any initial action” (“On the True Method in Philosophy and Theology” W 64 (1686)). If all things that exist are substances, and the essence of substance is activity, then passive “substances” do not truly exist, since they are passive, that is, not active (Shapere, 46).