If we were to admit the postulate of relativity, we would find the same number in the law of gravitation and the laws of electromagnetism—the speed of light—and we would find it again in all other forces of any origin whatsoever. This state of affairs may be explained in one of two ways: either everything in the universe would be of electromagnetic origin, or this aspect—shared, as it were, by all physical phenomena—would be a mere epiphenomenon, something due to our methods of measurement.  How do we go about measuring? The first response will be: we transport solid objects considered to be rigid, one on top of the other. But that is no longer true in the current theory if we admit the Lorentzian contraction. In this theory, two lengths are equal, by definition, if they are traversed by light in equal times.

Perhaps if we were to abandon this definition Lorentz’s theory would be as fully overthrown as was Ptolemy’s system by Copernicus’s intervention. Should that happen someday, it would not prove that Lorentz’s efforts were in vain, because regardless of what one may think, Ptolemy was useful to Copernicus.

M. H. Poincaré (1906). Sur la dynamique de l’électron. In: Rendiconti del Circolo matematico di Palermo 21.1, pp. 129–175. Translated by Scott Walter In J. Renn (ed.), The Genesis of General Relativity Vol. 3: Theories of Gravitation in the Twilight of Classical Physics; Part I (Boston Studies in the Philosophy of Science 201)