 Substances, according to Gottfried Wilhelm Leibniz (1646–1716), always act; furthermore, since even finite, created substances are naturally indestructible and thus immortal, substances continue to act forever. To what or to whom do substances causally owe their action? In Leibniz’s day, this question more or less becomes a question about the causal role of God. Is God the only genuine causal agent in nature? Or does God’s causal contribution, at least in the ordinary development of nature, consist “merely” in the creation and conservation of created substances? We will see that Leibniz holds that both God and created substances are causally responsible for changes in the states of substances. There is another question particularly salient for the seventeenth century philosopher: What kinds of causes are present in substantial activity? Just thirty years after Leibniz’s death, David Hume stated that his own definition of cause entails that “all causes are of the same kind,” namely, efficient (i.e., productive) causes. Leibniz’s theory of causation instead integrates efficient, final, and even formal causes, as it tries to explain away real causation among phenomenal things with his infamous pre-established harmony.