 Perhaps because he thinks he already has an account that will work for him—the pre-established harmony—Leibniz does not believe that a plausible theory of causation need be intersubstantial in kind. In other words, Leibniz does not face a problem so many of his contemporaries face: his metaphysics—unlike their own—includes the necessary (and sufficient?) elements to sustain a properly and purely intrasubstantial theory of causation; a Leibnizian created substance is self-sufficient (possessing an inner principle of change that explains all of its property-instances), permanently causally efficacious (acting does not diminish its power), and equipped with a complete concept that conforms to the complete concepts of all other created substances (perceiving or representing one and the same universe). Thus, it makes sense that Leibniz would not feel compelled to defend intersubstantial causation. Like reference to miracles, it would be an unnecessary move, and therefore methodologically invalid. 