Everything true of a substance is so because the predicate of a true proposition is contained in the complete concept of its subject.

Yet Leibniz often seems to avert such a conclusion by appeal to his idea of “pre-established harmony”, and this is possible because he himself interprets this idea in two different ways. Early in his career, in such texts as “Primary Truths” (1680–84) and the “Discourse on Metaphysics” (1686) (both texts unpublished in Leibniz’s lifetime and not known to his immediate successors such as Wolff and Baumgarten), Leibniz introduces the doctrine of pre-established harmony on truth-theoretical grounds