 Leibniz’s first reason for denying inter-substantial causation, that “one cannot explain how something can pass from one thing into the substance of another,” is a clear reference to the influx theory of causation. Leibniz found this theory inconsistent with his conception of substance. Influx theory could only explain causal relations between entities with parts, according to Leibniz. It is through these parts that “something can pass from one thing into … another.” But Leibnizian substances do not have parts in the requisite sense. (For more on influx theory, see O’Neil 1993.) Leibniz also gives a reason tied to his complete concept theory of substance, according to which “the nature of an individual substance or of a complete being is to have a notion so complete that it is sufficient to contain and to allow us to deduce from it all the predicates of the subject to which this notion is attributed” (Discourse on Metaphysics, sec. 8; (G IV, 433/A&amp;G 41)). But there are, it seems, at least two problems with this explanation. First, Leibniz moves rather quickly from a conceptual explanation of substance in terms of the complete concept theory to the conclusion that this consideration is sufficient to explain the activity of concrete substances. Second, even if conceptual considerations about substances were sufficient to explain their apparent causal activity, it does not seem to follow that substances do not interact—unless one is assuming that causal overdetermination is not a genuine possibility. Leibniz seems to be assuming just that but without argument. 