Richard Arthur (McMaster University, Hamilton, Ontario, Canada), in Substantial forms, points of view, and the relativity of motion, argues that one of Leibniz’s motivations for reintroducing substantial forms was to save the reality of motion. Already in 1676 Leibniz had established that motion, understood geometrically (i.e. as change of situation), is merely relative, and therefore a pure phenomenon or appearance. True motions, on the other hand, according to Leibniz, are identifiable by reference to their causes, and these are determined by appeal to the most intelligible hypothesis for understanding the phenomena. Arthur argues that the introduction of substantial forms, reinterpreted as enduring primitive forces of action in each corporeal substance, allows Leibniz to found the reality of the phenomena of motion in force, and thus avoid reducing motion to a mere appearance. Arthur argues that the entelechies of Leibniz’s mature philosophy continue to serve this same function, in opposition to the view that his middle-period realism gave way to an idealist stance on motion.