In April 1669, Leibniz wrote a letter to Jakob Thomasius in which he argues at length for the reconciliation of the Aristotelian and the mechanical philosophies and for a conception of substance that would effect that reconciliation. In the same year, he prepared an edition of a text by the sixteenth-century humanist, Mario Nizolio, which he published in early 1670. Leibniz wrote a lengthy introduction to Nizolio's book, On the True Principles and the True Method of Philosophizing, Against the Pseudo-Philosophers, of 1553. Both Nizolio's text and Leibniz's introduction discuss the proper way of philosophizing. It is significant that Leibniz attached to his introduction a slightly revised version of his April 1669 letter to Thomasius. The letter thereby became the young man's first published text on a contemporary metaphysical topic. Both Leibniz's proposal for reconciliation and his argument for it are strikingly odd. He insists that much of mechanical philosophy follows from Aristotelian principles and that Aristotle's physics is explicable in mechanical terms. His argument includes a reformation of Aristotle's notions of substantial form and matter into a mechanical conception of body. Leibniz happily concludes that by such means the mechanical philosophy “can be reconciled with Aristotle's.”

The apparent perversity of Leibniz's position is due to the fact that nothing could be further from the truth. The philosophies proposed by mechanists like Descartes and Gassendi explicitly reject the foundations of the Aristotelian system.