318
DOC.
71
PRINCETON LECTURES
THE GENERAL
THEORY
this
principle
is
evidently intimately
connected with
the
law
of
the
equality
between the inert and the
gravita-
tional
mass,
and
signifies
an
extension of
the
principle
of
relativity to
co-ordinate
systems
which
are
in non-uniform
motion
relatively
to
each other. In
fact, through
this
conception
we
arrive
at
the
unity
of
the
nature
of
inertia
and
gravitation.
For
according
to
our
way
of
looking
at
it,
the
same masses
may
appear
to
be either
under
the
action
of
inertia
alone
(with respect to
K) or
under
the
combined action
of
inertia and
gravitation (with
respect
to
K').
The
possibility
of
explaining
the numerical
equality
of
inertia and
gravitation
by
the
unity
of
their
nature
gives
to
the
general theory
of
relativity, according
to
my
conviction,
such
a
superiority
over
the
conceptions
of
classical
mechanics,
that
all
the
difficulties
encountered
must
be
considered
as
small in
comparison
with
this
progress.
What
justifies us
in
dispensing
with the
preference
for
inertial
systems
over
all
other co-ordinate
systems,
a
preference
that
seems so
securely
established
by experience?
The
weakness of
the
principle
of
inertia
lies
in
this,
that
it
involves
an
argument
in
a
circle:
a mass moves
without
acceleration
if
it
is
sufficiently
far
from
other
bodies;
we
know
that it
is
sufficiently
far from
other
bodies
only by
the
fact that it
moves
without
acceleration.
Are
there
at
all
any
inertial
systems
for
very
extended
portions
of the
space-time continuum,
or,
indeed, for
the
whole universe?
We
may
look
upon
the
principle
of
inertia
as
established,
to
a
high degree
of
approximation,
for the
space
of
our
planetary
system,
provided
that
we
neglect
the
perturba-
tions
due
to
the
sun
and
planets.
Stated
more
exactly,
there
are
finite
regions, where,
with
respect
to
a
suitably
chosen
space
of
reference,
material
particles
move
freely
[58]
[73]
[74]
[75]
[76]
[77]
[78]