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But the considerations of Sections XXV and XXVI show us the way

to surmount this difficulty. We refer the four-dimensional

space-time continuum in an arbitrary manner to Gauss

co-ordinates. We assign to every point of the continuum (event)

four numbers, x1, x2, x3, x4 (co-ordinates), which have not the

least direct physical significance, but only serve the purpose of

numbering the points of the continuum in a definite but arbitrary

manner. This arrangement does not even need to be of such a kind

that we must regard x1, x2, x3, as "space" co-ordinates and x4,

as a "time" co-ordinate.