the view that space is physically empty. But therewith the
conception of the ether has again acquired an intelligible content,
although this content differs widely from that of the ether of the
mechanical undulatory theory of light. The ether of the general
theory of relativity is a medium which is itself devoid of _all_
mechanical and kinematical qualities, but helps to determine
mechanical (and electromagnetic) events.

What is fundamentally new in the ether of the general theory of
relativity as opposed to the ether of Lorentz consists in this, that
the state of the former is at every place determined by connections
with the matter and the state of the ether in neighbouring places,
which are amenable to law in the form of differential equations;
whereas the state of the Lorentzian ether in the absence of
electromagnetic fields is conditioned by nothing outside itself,
and is everywhere the same. The ether of the general theory of
relativity is transmuted conceptually into the ether of Lorentz if
we substitute constants for the functions of space which describe
the former, disregarding the causes which condition its state.
Thus we may also say, I think, that the ether of the general theory
of relativity is the outcome of the Lorentzian ether, through
relativation.

As to the part which the new ether is to play in the physics of
the future we are not yet clear. We know that it determines the
metrical relations in the space-time continuum, e.g. the configurative
possibilities of solid bodies as well as the gravitational fields;
but we do not know whether it has an essential share in the structure
of the electrical elementary particles constituting matter. Nor do
we know whether it is only in the proximity of ponderable masses