Contrary to popular belief, I argue that Leibniz is not hopelessly confused about motion: Leibniz is indeed both a relativist and an absolutist about motion, as suggested by the textual evidence, but, appearances to the contrary, this is not a problem; Leibniz’s infamous doctrine of the equivalence of hypotheses is well-supported and well-integrated within Leibniz’s physical theory; Leibniz’s assertion that the simplest hypothesis of several equivalent hypotheses can be held to be true can be explicated in such a way that it makes good sense; the mere Galilean invariance of Leibniz’s conservation law does not compromise Leibniz’s relativism about motion; and Leibniz has a straightforward response to Newton’s challenge that the observable effects of the inertial forces of rotational motions empirically distinguish absolute from relative motions.

In this paper I challenge the common view that early in his career (1679-1695) Leibniz held that space and time are well-founded phenomena, entities on an ontological par with bodies and their properties. I argue that the evidence Leibniz ever held that space and time are well-founded phenomena is extremely weak and that there is a great deal of evidence for thinking that in the 1680s he held a position much like the one scholars rightly attribute to him in his mature period, namely, that space and time are merely orders of existence and as such are purely abstract and occupy an ontological realm distinct from that of well-founded phenomena. In the course of arguing for this interpretation, I offer an account of the nature of Leibnizian phenomena which allows Leibniz to hold the view that space and time are phenomena, while at the same time thinking of them as abstract, ideal orders of existence.

This paper examines the young Kant’s claim that all motion is relative, and argues that it is the core of a metaphysical dynamics of impact inspired by Leibniz and Wolff. I start with some background to Kant’s early dynamics, and show that he rejects Newton’s absolute space as a foundation for it. Then I reconstruct the exact meaning of Kant’s relativity, and the model of impact he wants it to support. I detail (in Section II and III) his polemic engagement with Wolffian predecessors, and how he grounds collisions in a priori dynamics. I conclude that, for the young Kant, the philosophical problematic of Newton’s science takes a back seat to an agenda set by the Leibniz-Wolff tradition of rationalist dynamics. This results matters, because Kant’s views on motion survive well into the 1780s. In addition, his doctrine attests to the richness of early modern views of the relativity of motion.