 So far (apart from that remark about ‘unchanged’ bodies) we have not seen Leibniz introduce anything more than relations of distance between bodies, which is certainly consistent with the folk view of his philosophy. However, Leibniz sought to provide a foundation for the Cartesian/mechanical philosophy in terms of the Aristotelian/scholastic metaphysics of substantial forms (here we discuss the views laid out in Sections 17–22 of the 1686 Discourse on Metaphysics and the 1695 Specimen of Dynamics, both in Garber and Ariew 1989). In particular, he identifies primary matter with what he calls its ‘primitive passive force’ of resistance to changes in motion and to penetration, and the substantial form of a body with its ‘primitive active force’. It is important to realize that these forces are not mere properties of matter, but actually constitute it in some sense, and further that they are not themselves quantifiable. However, because of the collisions of bodies with one another, these forces ‘suffer limitation’, and ‘derivative’ passive and active forces  result.[11]  Derivative passive force shows up in the different degrees of resistance to change of different kinds of matter (of ‘secondary matter’ in scholastic terms), and apparently is measurable. Derivative active force, however, is considerably more problematic for Leibniz. On the one hand, it is fundamental to his account of motion and theory of mechanics – motion fundamentally is possession of force. But on the other hand, Leibniz endorses the mechanical philosophy, which precisely sought to abolish Aristotelian substantial form, which active force represents. Leibniz’s goal was to reconcile the two philosophies, by providing an Aristotelian metaphysical foundation for modern mechanical science; as we shall see, it is ultimately an open question exactly how Leibniz intended to deal with the inherent tensions in such a view.