 These equations express relations, and if the equations remain true it is because these relations preserve their reality. They teach us, before and after, that there is such and such a relation between some thing and some other thing; only this something we used to call motion, we now call it electric current. But these names were only images substituted for the real objects which nature will eternally hide from us. The true relations between these real objects are the only reality we can attain to, and the only condition is that the same relations exist between these objects as between the images by which we are forced to replace them. If these relations are known, what does it matter if we deem it convenient to replace one image by another? (1900: 15, author’s translation.