 However, the above texts aside, there is a philosophical worry with attributing efficiency or productivity to perceptions on Leibniz’s metaphysics. The question is whether perceptions can function as active agencies (and so efficient causes) for Leibniz. In Leibniz’s ontology, what is active is a true, enduring unity and a center of force—namely, a substance. Leibniz writes that “everything that acts is an individual substance” (“On Nature Itself” AG 160). To understand perceptions and perceptual states as causally ordered, in the sense that efficient causal and not merely lawful or regular or final causal relations obtain among them, is to subscribe to an event oriented model of causation. But Leibniz, not only like his medieval predecessors but also many of his contemporaries, subscribes to a rather different model—one in which a real cause must be a substance or a power of that substance. (More on this in the section on causal models.) Therefore, unless perceptions are genuine substances and so not events, it seems to follow that for Leibniz, perceptions (and perceptual states) are causally inefficacious.