DOC. 38 ETHER AND RELATIVITY 175
ETHER
AND RELATIVITY
17
to be able to look
upon
the rotation
of
the
system,
at
least
formally,
as some-
thing
real,
Newton
objectivises
space.
Since he classes his absolute
space together
with
real
things,
for him
rotation relative
to
an
absolute
space
is also
something
real. Newton
might
no
less well
have
called his absolute
space
“Ether”;
what
is essential is
merely
that
besides
observ-
able
objects,
another
thing,
which is
not
perceptible,
must
be looked
upon
as
real,
to enable acceleration
or
rotation
to
be looked
upon
as
something
real.
It
is true
that
Mach
tried
to avoid
having
to
accept
as
real
something
which
is not observable
by
endeavouring
to
substitute
in mechanics
a mean
acceler-
ation with
reference to
the
totality
of
the
masses
in
the
universe in
place
of
an
[18]
acceleration
with
reference
to
absolute
space.
But
inertial
resistance
opposed
to
relative
acceleration
of
distant
masses
presupposes
action at
a
distance; and
as
the
modern
physicist
does not believe
that
he
may
accept
this
action at
a
dis-
tance,
he
comes
back
once more,
if he