 Yet this early model of causation possesses at least one virtue that the contemporary one does not. It has the distinct advantage of avoiding any problematic regress of causal explanation. Once a causal sequence of events is laid out, we can always ask why that sequence, and not another, occurred. But an explanation of this sequence, according to an event/event model of causation does not preclude the same question “Why that sequence of events?” being asked of it again and again. On the other hand, Leibniz does not think that the power or will, which are not events at all, but faculties, of an agency or agent, need any explaining. The regress of explanation stops abruptly with the substance. In a letter to De Volder (June 30, 1704), he writes: “To … ask why there is perception and appetite in simple substances is to inquire about something ultramundane, so to speak, and to demand reasons of God why he has willed things to be such as we conceive them to be” (GP ii, 271/L 538). Furthermore, the central message of Leibniz’s essay, Against Barbaric Physics (c. 1713), seems to be that since the actions of substances are in fact spontaneous, there is no way of explaining change in monads unless we posit some force or power that drives substantial action. If it turns out that substances just are forces or powers, as Julia Jorati and John Whipple maintain, it is no violation of this causal model. At any rate, without this agency in monads, there would be no cause sufficient to determine the whole sequence of events and thus no adequate causal explanation of the totality of actions (i.e., perceptions) on the part of substances. And, besides failing to break out of the cycle of explanation that appears to plague event/event models of causation, such a picture appears to violate Leibniz’s Principle of Sufficient Reason: