Thus, confining ourselves to the two places and , there are two distinct processes of correlating the time-order of events throughout the universe, namely by a series of observations of coincidences at based on time-order and by a series of observations of coincidences at based on time-order . These two processes are distinct and will only agree by some accident of special circumstance. 11.2 What are the observations at which will assign to an event at a position in the time-order ? Suppose some message—a wave disturbance, for example—starts from when event happens at , reaches when event happens at and is immediately reflected so as to return to when event ″ happens at . Now according to the method of time-measurement for , there is an event ′ which happens at mid-time between and ″. Then, when certain conditions have been fulfilled, the event at is defined as simultaneous with the event ′ at according to the method of correlation appropriate to place . In this way a time-order of events at is derived solely from observation of coincidence at and is based solely on the fundamental time-order at . Thus the time-order at is extended as a time-order for all events at all places. 11.3 There are questions which require elucidation before this definition can be understood. What is a place? We have chosen a vague term on purpose, so as to postpone its consideration until now. A place can only be marked by phenomena capable of recognition, for example the continued appearance of a material body. Thus we must construe and to be the names of material bodies, or of persistent sets of circumstances which will serve the same purpose. In general and will be in relative motion with respect to each other. What of the message which passes from to and back to ? Its transmission must be uniform. Suppose the message travels with velocity , that is, with the velocity of light in vacuo. Then, assuming the electromagnetic formulae for relativity, this velocity relative to is independent (so far as its magnitude is concerned) of the velocity which we ascribe to through space. 11.4 Thus our recording body can be any body at rest in some consentient set of the Newtonian group, and we reckon motion as relative to the space of this set. We send our message with the velocity of light in vacuo. Then, according to the local time-order at , the event at is simultaneous with the event ′ at . This definition of simultaneity in the local time-order at is independent of any assumption of absolute rest for provided that the electromagnetic formulae for relativity are adopted. The local time-order at is also in complete agreement with the local time-order at any body which is rigidly connected with , i.e. which belongs to the same consentient set.