Influx theory can only explain causal relations between entities that have parts, since it is through parts that something passes from one thing into another.

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