DOC. 42
SPECIAL AND GENERAL RELATIVITY
395
156
Relativity
It
is
indeed
an exacting
requirement to
have
to
ascribe
physical
reality to
space
in
general,
and
especially
to
empty
space.
Time
and
again
since
remotest
times
philosophers
have
resisted such
a
presumption.
Descartes
argued
somewhat
on
these lines:
space
is
identical with
extension,
but
extension
is
connected with
bodies;
thus
there
is
no
space
without bodies
and hence
no
empty space.
The
weakness of this
argument
lies
primarily
in what
follows.
It
is
certainly true
that the
concept
extension
owes
its
origin
to
our experiences
of
laying
out
or bringing
into
contact solid
bodies. But from this it
cannot
be concluded
that
the
concept
of
extension
may not
be
justified
in
cases
which have
not
themselves
given
rise
to
the
formation of this
concept.
Such
an
enlargement
of
concepts
can
be
justified indirectly by
its
value
for
the
comprehension
of
empirical
results.
The
assertion that
extension
is
confined
to
bodies
is
therefore of itself
certainly
unfounded.
We
shall
see
later, however,
that the
general theory
of
relativity
con-
firms
Descartes'
conception
in
a
roundabout
way.
What
brought
Descartes
to
his
remarkably
attractive view
was cer-
tainly
the
feeling
that,
without
compelling necessity,
one
ought
not
to
ascribe
reality
to
a
thing
like
space,
which
is not
capable
of
being "directly experienced."1
The
psychological origin
of the idea
of
space,
or
of the
necessity
for
it, is
far from
being
so
obvious
as
it
may appear
to
be
on
the
basis of
our
customary
habit of
thought.
The
old
geometers
deal with
conceptual objects (straight
line,
point,
surface),
but
not really
with
space
as
such,
as was
done later in
1
This
expression is to
be taken
cum
grano salis.