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is now subjected to the moving force-vector of m alone. If we now modify this given vector, first by writing ${\dot {t}}^{-1}={\sqrt {1-{\frac {v^{2}}{c^{2}}}}}$, which = 1 up to magnitudes of the order $1/c^{2}$, then it is shown^[1] that Kepler's laws hold good for the positions $x_{1},y_{1},z_{1}$ of $m_{1}$ at any time, except that in place of the times $t_{1}$ we have to write the proper times $\tau _{1}$ of $m_{1}$. On the basis of this simple remark, it can be seen that the proposed law of attraction in combination with new mechanics is not less suited for the explanation of astronomical phenomena than the Newtonian law of attraction in combination with Newtonian mechanics.

Also the fundamental equations for electromagnetic processes in ponderable bodies are in accordance with the world-postulate throughout. I shall also show on a later occasion that even the deduction of these equations, as taught by Lorentz on the basis of the concepts of electron theory, are by no means to be given up.

The validity of the world-postulate without exception is, I would like to believe, the true core of an electromagnetic picture of the world; this core was first hit upon by Lorentz, further carved out by Einstein, bringing it completely to daylight. In course of further developing the mathematical consequences, enough suggestions will be forthcoming for the experimental verification of the postulate; in this way even those, who find it unsympathetic or even painful to give up the old, time-honoured concepts, will be reconciled by idea of a pre-established harmony between pure mathematics and physics.
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1. H. Minkowski, l.c., p. 110.