 What exactly causes the change of perceptions of a substance? Leibniz holds that both the “primitive active power” of a substance and its perceptions or perceptual states play crucial causal roles in the changes of a substance. (As we shall see, furthermore, God is no minor player in this effort as well.) The primitive active power of a substance, Leibniz tells us, is “a nature or an internal force that can produce in it, in an orderly way … all the appearances or expressions it will have, without the help of any created being” (New System GP iv, 486/AG 144). Moreover, “the present state of each substance is a natural result [consequence] of its preceding state” (Clarification of Difficulties concerning Monsieur Bayle GP iv, 521).