"In the long debate about action at a distance versus contact action we also find a concern with the effective individuation of particles and subsystems and with distant action. Belief in contact action goes back to Aristotle and Eudoxes, based on the self-evident regulative principle that a thing cannot act where it is not. For them, causal explanations were essential for true science. For Descartes too, there could be no vacuum and what may appear to be empty is actually filled with an aether. After the Principia, there were basically two camps on the "gravity dilemma." The one, among whose members were the Cartesians, Leibniz, and Huygens, maintained that an ether was required to allow any intelligible explanation of gravitational phenomena. The other, notably represented by Newton's ally... , took gravity to be evidence for God's action. ...God is everywhere and, hence, "instantaneous" action... is no mystery. The basic motivating factor in demanding contact action was that of intelligibility. Maxwell put the matter quite succinctly when he argued for "a force of the old school—a case of vis a tergo—a shove from behind."