Abstract: Influential interpreters have held that Leibniz's extensive use of ostensibly Aristotelian concepts of substantial form and primary matter during his “middle years” (before about 1704) present a philosophy that is less purely a monadology or form of idealism than it later became. This chapter argues, to the contrary, (1) that Leibniz's substantial forms are assimilated not only to forces but also (and more sweepingly than in Scholasticism) to souls and (2) that interesting arguments of the middle years, in which Leibniz criticizes Descartes's conception of corporeal substance, leave no room for “primary matter” to be anything more than an aspect of perceiving substances.

Keywords: Aristotle, corporeal substance, Descartes, forces, idealism, Leibniz, primary matter, Scholasticism, souls, substantial form,