 Mach’s proposal had the advantage of a clear empirical motivation; Neumann’s “body Alpha” seemed no less mysterious than absolute space, and almost sounds comical to the modern reader. But Neumann’s discussion of a time-scale was somewhat more fruitful, and employed the principle that Euler had already expressed (1748): the law of inertia defines a time-scale, by which equal intervals of time are those in which a free particle travels equal distances. He also noted, however, that this definition is quite arbitrary. For, in the absence of a prior definition of equal times, any motion whatever can be stipulated to be uniform. It is no help to appeal to the requirement of freedom from external forces, since the free particles presumably are known to us only by their uniform motion. We have a genuine empirical claim only when we state of at least two free particles that their motions are mutually proportional. Equal intervals of time can then be defined as those in which two free particles travel mutually proportional distances.