Although Leibniz was an ontological monist who believed that only one type of substance, the monad, exists in the universe, and that everything is reducible to it, he nonetheless maintained that there was an important distinction between "the mental" and "the physical" in terms of causation.
Selvom Leibniz var en ontologisk monist, som mente at der kun eksisterer en enkelt type substans i universet, monaden, og at alt kan reduceres til den, fastholdt han dog alligevel at der er en vigtig skelnen mellem "det mentale" og "det fysiske" hvad angår årsagssammenhæng.