 Between the time of Newton and Leibniz and the 20th century, Newton’s mechanics and gravitation theory reigned essentially unchallenged, and with that long period of dominance, Newton’s absolute space came to be widely accepted. At least, no natural philosopher or physicist offered a serious challenge to Newton’s absolute space, in the sense of offering a rival theory that dispenses with it. But like the action at a distance in Newtonian gravity, absolute space continued to provoke philosophical unease. Seeking a replacement for the unobservable Newtonian space, Neumann (1870) and Lange (1885) developed more concrete definitions of the reference frames in which Newton’s laws   hold.[1]  In these and a few other works, the concept of the set of inertial frames (those in which material bodies obey Newton’s three laws of motion) was first clearly expressed, though it was implicit in both remarks and procedures found in Newton’s Principia. (See the entries on  space and time: inertial frames  and  Newton’s views on space, time, and motion)  The most sustained, comprehensive, and influential attack on absolute space was made by Ernst Mach in his Science of Mechanics (1883).