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.
Meski Leibniz adalah seorang monis ontologis yang meyakini bahwa hanya ada satu jenis substansi saja (monad) di alam semesta, dan bahwa semuanya dapat disusutkan ke monad, ia menyatakan bahwa ada pemisahan antara "budi" dan "fisik" dalam ranah sebab-musabab.