After further consideration you cast a somewhat disdainful glance at me—and rightly so—and you declare: “I maintain my previous definition nevertheless, because in reality it assumes absolutely nothing about light. There is only _one_ demand to be made of the definition of simultaneity, namely, that in every real case it must supply us with an empirical decision as to whether or not the conception that has to be defined is fulfilled. That my definition satisfies this demand is indisputable. That light requires the same time to traverse the path _A_ → _M_ as for the path _B_ → _M_ is in reality neither a _supposition nor a hypothesis_ about the physical nature of light, but a _stipulation_ which I can make of my own freewill in order to arrive at a definition of simultaneity.” It is clear that this definition can be used to give an exact meaning not only to _two_ events, but to as many events as we care to choose, and independently of the positions of the scenes of the events with respect to the body of reference[7] (here the railway embankment). We are thus led also to a definition of “time” in physics. For this purpose we suppose that clocks of identical construction are placed at the points _A, B_ and _C_ of the railway line (co-ordinate system) and that they are set in such a manner that the positions of their pointers are simultaneously (in the above sense) the same. Under these conditions we understand by the “time” of an event the reading (position of the hands) of that one of these clocks which is in the immediate vicinity (in space) of the event. In this manner a time-value is associated with every event which is essentially capable of observation. [7] We suppose further that, when three events _A, B_ and _C_ occur in different places in such a manner that, if _A_ is simultaneous with _B_, and _B_ is simultaneous with _C_ (simultaneous in the sense of the above definition), then the criterion for the simultaneity of the pair of events _A, C_ is also satisfied. This assumption is a physical hypothesis about the law of propagation of light; it must certainly be fulfilled if we are to maintain the law of the constancy of the velocity of light _in vacuo_. This stipulation contains a further physical hypothesis, the validity of which will hardly be doubted without empirical evidence to the contrary. It has been assumed that all these clocks _go at the same rate_ if they are of identical construction. Stated more exactly: When two clocks arranged at rest in different places of a reference-body are set in such a manner that a _particular_ position of the pointers of the one clock is _simultaneous_ (in the above sense) with the _same_ position, of the pointers of the other clock, then identical “settings” are always simultaneous (in the sense of the above definition). IX. THE RELATIVITY OF SIMULTANEITY