 Thomson did not reject the term “absolute rotation”. Instead, he held that it is properly defined as rotation relative to a frame that satisfies his definition of a reference frame. A body that is rotating with respect to a reference frame (and dial-traveller) is rotating with respect to any other frame in uniform motion relative to the first. The definition does not express, as Lange’s does, the degree of arbitrariness involved in the construction of an inertial system by means of free particles. By dispensing with the idealization of free particles, Thomson’s definition aims to characterize an inertial frame for an actual system of interacting bodies. However, it does not quite fulfill its aim. Like Lange’s definition, it leaves out a crucial condition for an inertial system as we understand it: all forces must belong to action-reaction pairs. Otherwise we could have, as on a rotating sphere, merely apparent (centrifugal) forces that are, by definition, proportional to mass and acceleration, and so the rotating sphere would satisfy Thomson’s definition. Therefore the definition needs to be completed by the stipulation that to every action there is an equal and opposite reaction. (This completion was actually proposed by R.F. Muirhead in 1887.) 