It may seem strange that we should be able to deduce the contraction of a material rod and the retardation of a material clock from the general geometry of space and time. But it must be remembered that the contraction and retardation do not imply any absolute change in the rod and clock. The ``configuration of events'' constituting the four-dimensional structure which we call a rod is unaltered; all that happens is that the observer's space and time partitions cross it in a different direction. \PageSep{27} Further we make no prediction as to what would happen to the rod set in motion in an actual experiment. There may or may not be an absolute change of the configuration according to the circumstances by which it is set in motion. Our results apply to the case in which the rod after being set in motion is (according to all experimental tests) found to be similar to the rod in its original state of rest\footnotemark.\footnotetext {It may be impossible to change the motion of a rod without causing a rise of temperature. Our conclusions will then not apply until the temperature has fallen again, i.e.\ until the temperature-test shows that the rod is precisely similar to the rod before the change of motion.} When a number of phenomena are connected together it becomes somewhat arbitrary to decide which is to be regarded as the explanation of the others. To many it will seem easier to regard the strange property of the fundamental velocity as \emph{explained} by these differences of behaviour of the observers' clocks and scales. They would say that the observers arrive at the same value of the velocity of light because they omit the corrections which would allow for the different behaviour of their measuring-appliances. That is the relative point of view, in which the relative quantities, length, time, etc., are taken as fundamental. From the absolute point of view, which has regard to intervals only, the standards of the two observers are equal and behave similarly; the so-called \emph{explanations} of the invariance of the velocity of light only lead us away from the root of the matter. Moreover the recognition of the FitzGerald contraction does not enable us to avoid paradox. From~\Eq{(5.3)} we found that $S'$'s~longitudinal measuring-rods were contracted relatively to those of~$S$. From~\Eq{(5.1)} we can show similarly that $S$'s~rods are contracted relatively to those of~$S'$. There is complete reciprocity between $S$ and~$S'$. This paradox is discussed more fully in \Title{Space, Time and Gravitation}, p.~55. \Section{11.}{Simultaneity at different places} \index{Simultaneity at different places}% It will be seen from the fourth equation of~\Eq{(5.1)}, viz.\ \[ dt = -\beta (t' - ux'/c^{2}), \] that events at different places which are simultaneous for~$S'$ are not in general simultaneous for~$S$. In fact, if $dt' = 0$, \[ dt = -\beta u\, dx'/c^{2}. \Tag{(11.1)} \]