DOC. 38 ETHER AND RELATIVITY 163
ETHER
AND RELATIVITY
5
action
only
through
contact,
and
not
through
immediate action
at
a
distance.
It
is
only
with reluctance that man's
desire for
knowledge
endures
a
dualism
of
this
kind. How
was
unity
to be
pre-
served in his
comprehension
of
the
forces
of nature?
Either
by trying
to look
upon
contact
forces
as being
themselves
distant
forces which
admittedly
are
obser-
vable
only
at
a
very
small
distance-
and this
was
the road
which Newton's
followers,
who
were entirely
under the
spell
of his
doctrine, mostly
preferred
to
take;
or
by
assuming
that the
Newton-
ian action
at
a
distance is
only apparently
immediate action at
a
distance,
but in
truth
is
conveyed
by
a
medium
permeat-
ing space,
whether
by
movements
or
by
elastic
deformation of
this
medium. Thus
the
endeavour
toward
a
unified view
of
the
nature of forces leads
to the
hypothesis
of
an
ether. This
hypothesis,
to
be
sure,
did
not at first
bring
with
it
any
advance
in
the
theory
of
gravitation
or
in
physics
generally, so
that
it
became
customary
to
treat
Newton’s law of force
as an
axiom