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That is, by the afore-mentioned choice of "coordinates," (11a)

[see the end of Appendix II] is transformed into this equation.

We see from (12) that the imaginary time co-ordinate x4, enters

into the condition of transformation in exactly the same way as

the space co-ordinates x1, x2, x3. It is due to this fact that,

according to the theory of relativity, the "time" x4, enters into

natural laws in the same form as the space co ordinates x1, x2,

x3.

A four-dimensional continuum described by the "co-ordinates" x1,

x2, x3, x4, was called "world" by Minkowski, who also termed a

point-event a "world-point." From a "happening" in

three-dimensional space, physics becomes, as it were, an

"existence" in the four-dimensional "world."