DOC.
71
PRINCETON LECTURES 315
THE
GENERAL THEORY
OF RELATIVITY
[67]
ALL
of the
previous
considerations have been
based
upon
the
assumption
that
all
inertial
systems
are
equivalent
for
the
description
of
physical phenomena,
but
that
they
are
preferred,
for
the formulation
of
the
laws
of nature,
to
spaces
of
reference
in
a
different
state
of
motion.
We
can
think
of
no cause
for
this
preference
for
definite
states
of motion
to
all
others,
according to
our
previous considerations,
either
in
the
perceptible
bodies
or
in
the
concept
of
motion;
on
the
contrary,
it
must
be
regarded
as an
independent property
of
the
space-time
continuum.
The
principle
of
inertia,
in
particular,
seems
to
compel us
to
ascribe
physically
objective
properties
to
the
space-time
continuum.
Just
as
it
was
consistent
from the
Newtonian
standpoint to
make
both
the
statements,
tempus
est absolutum,
spatium
est
absolutum, so
from
the
standpoint
of the
special
theory
of
relativity
we
must
say,
continuum
spatii
et
temporis
est
absolutum.
In
this
latter
statement absolutum
means
not
only “physically
real,”
but also
“independent
in its
physical
properties, having
a
physical effect,
but
not
itself
influenced
by physical
conditions.”
As
long
as
the
principle
of inertia
is
regarded
as
the
keystone
of
physics,
this
standpoint is certainly
the
only
one
which
is
justified.
But
there
are
two
serious
criticisms
of the
ordinary conception.
In the
first
place,
it
is contrary
[55]