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 H. Poincare, Sur la dynamique 'electron, Rendiconti del circolo matematico di Palermo 21 (1906).

[P. C. M.]

Note 4. Relativity Theorem and Relativity-Principle.

Lorentz showed that the Maxwell-Lorentz system of electromagnetic field-equations remained practically unchanged by the Lorentz transformation. Thus the electromagnetic laws of Maxwell and Lorentz can be definitely proved "to be independent of the manner in which they are referred to two coordinate systems which have a uniform translatory motion relative to each other." (See "Electrodynamics of Moving Bodies," page 5.) Thus so far as the electromagnetic laws are concerned, the principle of relativity can be proved to be true. But it is not known whether this principle will remain true in the case of other physical laws. We can always proceed on the assumption that it does remain true. Thus it is always possible to construct physical laws in such a way that they retain their form when referred to moving coordinates. The ultimate ground for formulating physical laws in this way is merely a subjective conviction that the principle of relativity is universally true. There is no a priori logical necessity that it should be so. Hence the Principle of Relativity (so far as it is applied to phenomena other than electromagnetic) must be regarded as a postulate, which we have assumed to be true, but for which we cannot adduce any definite proof, until after the generalisation is made and its consequences tested in the light of actual experience. [P. C. M.]  Note 5.

See "Electrodynamics of Moving Bodies," p. 5-8.