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The only statements having regard to these points which can claim

a physical existence are in reality the statements about their

encounters. In our mathematical treatment, such an encounter is

expressed in the fact that the two lines which represent the

motions of the points in question have a particular system of

co-ordinate values, x1, x2, x3, x4, in common. After mature

consideration the reader will doubtless admit that in reality

such encounters constitute the only actual evidence of a

time-space nature with which we meet in physical statements.