DOC. 42 SPECIAL AND GENERAL RELATIVITY 403
164
Relativity
between
all
material
points.
(He made this
attempt
in order
to
arrive
at
a
satisfactory understanding
of
inertia).
The
Field
In Newtonian
mechanics, space
and time
play
a
dual
role.
First, they
play
the
part
of
carrier
or
frame
for
things
that
happen
in
physics,
in reference
to
which
events are
described
by
the
space
co-ordinates and
the
time. In
principle, matter
is
thought
of
as
consisting
of
"material
points,"
the
motions
of
which constitute
physical happening.
When
matter
is
thought
of
as
being
continuous,
this
is
done
as
it
were
provisionally
in
those
cases
where
one
does
not
wish
to
or
cannot
describe
the
discrete
structure.
In this
case
small
parts (elements
of
vol-
ume)
of the
matter
are
treated
similarly to
material
points,
at
least in
so
far
as we
are
concerned
merely
with
motions and
not
with
occurrences
which,
at
the
moment,
it is
not
possible
or serves no
useful
purpose to
attribute
to
motion
(e.g.
tem-
perature changes,
chemical
processes).
The
second
role
of
space
and time
was
that
of
being
an
"inertial
system."
From
all
conceivable
systems
of
reference,
inertial
systems
were
considered
to
be
advantageous
in
that,
with
respect to them,
the
law
of inertia claimed
validity.
In
this,
the
essential
thing
is
that
"physical reality,"
thought
of
as
being
independent
of the
subjects experiencing it,
was
conceived
as
consisting, at
least
in
principle,
of
space
and time
on one
hand,
and of
permanently existing
material
points,
moving
with
respect to space
and
time,
on
the
other.
The
idea
of
the
independent
existence
of
space
and time
can
be
ex-