How does it come about that alongside of the idea of ponderable matter, which is derived by abstraction from everyday life, the physicists set the idea of the existence of another kind of matter, the ether?
… according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether.
Thus we may also say, I think, that the ether of the general theory of relativity is the outcome of the Lorentzian ether. . . .
But this ether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it.