abstract:Gottfried Leibniz's theory of pre-established harmony () is a philosophical theory about causation under which every "substance" only affects itself, but all the substances (both bodies and minds) in the world nevertheless seem to causally interact with each other because they have been programmed by God in advance to "harmonize" with each other. Leibniz's term for these substances was "monads" which he described in a popular work (Monadology §7) as "windowless".