of matter are also, in their essence, nothing else than condensations
of the electromagnetic field, our present view of the universe
presents two realities which are completely separated from each other
conceptually, although connected causally, namely, gravitational ether
and electromagnetic field, or--as they might also be called--space
and matter.

Of course it would be a great advance if we could succeed in
comprehending the gravitational field and the electromagnetic field
together as one unified conformation. Then for the first time the
epoch of theoretical physics founded by Faraday and Maxwell would
reach a satisfactory conclusion. The contrast between ether and
matter would fade away, and, through the general theory of relativity,
the whole of physics would become a complete system of thought,
like geometry, kinematics, and the theory of gravitation. An
exceedingly ingenious attempt in this direction has been made by
the mathematician H. Weyl; but I do not believe that his theory will
hold its ground in relation to reality. Further, in contemplating
the immediate future of theoretical physics we ought not unconditionally
to reject the possibility that the facts comprised in the quantum
theory may set bounds to the field theory beyond which it cannot
pass.

Recapitulating, we may say that according to the general theory of
relativity space is endowed with physical qualities; in this sense,
therefore, there exists an ether. According to the general theory
of relativity space without ether is unthinkable; for in such space
there not only would be no propagation of light, but also no possibility
of existence for standards of space and time (measuring-rods and
clocks), nor therefore any space-time intervals in the physical