Now before the advent of the theory of relativity it had always tacitly been assumed in physics that the statement of time had an absolute significance, _i.e._ that it is independent of the state of motion of the body of reference. But we have just seen that this assumption is incompatible with the most natural definition of simultaneity; if we discard this assumption, then the conflict between the law of the propagation of light _in vacuo_ and the principle of relativity (developed in Section VII) disappears. We were led to that conflict by the considerations of Section VI, which are now no longer tenable. In that section we concluded that the man in the carriage, who traverses the distance _w per second_ relative to the carriage, traverses the same distance also with respect to the embankment _in each second_ of time. But, according to the foregoing considerations, the time required by a particular occurrence with respect to the carriage must not be considered equal to the duration of the same occurrence as judged from the embankment (as reference-body). Hence it cannot be contended that the man in walking travels the distance _w_ relative to the railway line in a time which is equal to one second as judged from the embankment. Moreover, the considerations of Section VI are based on yet a second assumption, which, in the light of a strict consideration, appears to be arbitrary, although it was always tacitly made even before the introduction of the theory of relativity. X. ON THE RELATIVITY OF THE CONCEPTION OF DISTANCE Let us consider two particular points on the train [8] travelling along the embankment with the velocity _v_, and inquire as to their distance apart. We already know that it is necessary to have a body of reference for the measurement of a distance, with respect to which body the distance can be measured up. It is the simplest plan to use the train itself as reference-body (co-ordinate system). An observer in the train measures the interval by marking off his measuring-rod in a straight line (_e.g._ along the floor of the carriage) as many times as is necessary to take him from the one marked point to the other. Then the number which tells us how often the rod has to be laid down is the required distance. [8] _e.g._ the middle of the first and of the hundredth carriage.