DOC. 30 FOUNDATION OF
GENERAL RELATIVITY
149
be
admitted
as
epistemologically
satisfactory,*
unless the
reason
given
is
an
observable fact
of experience.
The
law
of
causality
has
not
the
significance
of
a
statement
as
to
the
world of
experience,
except
when
observable
facts ultimately
appear
as causes
and
effects.
Newtonian mechanics
does not
give
a
satisfactory
answer
to
this
question.
It
pronounces
as
follows:-The
laws of
mechanics
apply
to
the
space R1,
in
respect
to
which the
body
S1
is at
rest,
but not
to
the
space
R2,
in
respect
to which
the
body
S2
is at rest.
But
the
privileged space
R1
of
Galileo,
thus
introduced,
is
a
merely factitious cause,
and
not
a
thing
that
can
be observed.
It
is
therefore
clear
that Newton's
mechanics
does
not
really satisfy
the
requirement
of
causality
in
the
case
under
consideration,
but
only
apparently
does
so,
since
it
makes
the factitious
cause R1
responsible
for
the
ob-
servable difference
in the
bodies
S1
and
S2.
The
only
satisfactory
answer
must
be
that the
physical
system consisting
of
S1
and
S2
reveals
within
itself
no imagin-
able
cause
to
which the
differing
behaviour
of
S1
and
S2
can
be
referred.
The
cause
must
therefore
lie
outside
this
system.
We
have to
take it that the
general
laws
of motion,
which in
particular
determine the
shapes
of
S1
and
S2,
must be
such
that
the mechanical
behaviour
of
S1
and
S2
is
partly
con-
ditioned,
in
quite
essential
respects,
by
distant
masses
which
we
have
not
included in the
system
under
consideration.
These distant
masses
and their motions
relative to
S1
and
S2
must
then
be
regarded
as
the
seat of
the
causes
(which
must be
susceptible
to
observation)
of
the
different
behaviour
of
our
two bodies
S1
and
S2.
They
take
over
the
role of
the
factitious
cause
R1.
Of all
imaginable
spaces
R1, R2,
etc.,
in
any
kind
of
motion
relatively
to
one
another,
there
is
none
which
we
may
look
upon
as privileged
a
priori
without
re-
viving
the
above-mentioned
epistemological objection.
The
laws
of
physics
must
be
of such
a
nature
that
they
apply to
systems
of
reference
in
any
kind
of motion.
Along
this road
we
arrive
at
an
extension
of
the
postulate
of relativity.
In addition
to
this
weighty argument
from the
theory
of
* Of
course an answer may
be
satisfactory
from
the
point
of
view
of
episte-
mology,
and
yet
be unsound
physically,
if it
is
in conflict with other
experi-
ences.
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