 So, for Leibniz, the essence of a created substance is activity, in the sense of being continually in process of change of its perceptions (Letter to De Volder, January 21, 1704, GP ii, 263/L 534). The accidents (tropes or property-instances) of a substance are its perceptions. The change in a created substance must be caused (Primary Truths C 519/L 268), yet there can be no transeunt (i.e., intersubstantial) causation (Discourse on Metaphysics §14, GP iv, 439/L 312/WF 66f). For causal interaction between created substances (i.e., monads) is in principle inexplicable (New System GP iv, 483/L 457) as seen earlier. Therefore, the cause of change of perceptions in a created substance is either to be located in an uncreated substance, namely God, or in the substance itself. But, as we have already seen, Leibniz rejects the view that God alone is the real causal power driving change in substances; as previously shown, he is no occasionalist (Notes on a Reply of Foucher, GP i, 373f/L 155). So, putting God aside for the moment, perceptual change is caused by the substance itself. But surely we want to know what it is about the substance or in the substance that drives perceptual change.