DOC. 42 SPECIAL AND GENERAL RELATIVITY 317
74
Relativity
It
is
true
that this
important
law had hitherto been recorded
in
mechanics,
but
it had
not
been
interpreted.
A
satisfactory
in-
terpretation
can
be obtained
only
if
we
recognise
the
following
fact:
The
same
quality
of
a
body
manifests
itself
according
to
cir-
cumstances
as
"inertia"
or as
"weight" (lit. "heaviness").
In
the
following
section
we
shall show
to
what
extent
this
is
ac-
tually
the
case,
and how this
question
is
connected with the
general postulate
of
relativity.