. This rule is definite because of the ultimate distinction between antecedence and subsequence in time, which has not otherwise been made use of. This standard choice of positive directions along mutual axes for two time-systems will always be adopted.
[50.3] The principle of kinematic symmetry has two parts, enunciating consequences which flow from the fact that the time-units in two time-systems
and
are congruent. The first part may be taken as the definition, or necessary and sufficient test, of such congruence.
The first part of the principle can be enunciated as the statement that the measures of relative velocities [i.e. the velocity of
in