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proportional to it. Under these conditions, the natural laws

satisfying the demands of the (special) theory of relativity

assume mathematical forms, in which the time co-ordinate plays

exactly the same role as the three space co-ordinates. Formally,

these four co-ordinates correspond exactly to the three space

co-ordinates in Euclidean geometry. It must be clear even to the

non-mathematician that, as a consequence of this purely formal

addition to our knowledge, the theory perforce gained clearness

in no mean measure.

[14] Cf. the somewhat more detailed discussion in Appendix II.