306
DOCS.
314,
315 MARCH
1917
level
every
uninterrupted
line
is straight.
The
correct derivation
proceeds
as
follows:
In
principle, finite
(matter-free)
portions
of
the world
can
exist for
which,
on
suitable
choice of frame of
reference,
ds2
=
dX¡
+

+

-
dX\
applies.
(If
this
were
not
the
case,
the Galilean
law of
inertia and the
special
theory
of rel. could
not
be
upheld.)
In such
a
segment
of
the
world,
with
this
reference
system
choice,
the
Galilean
law
of
inertia
is
valid,
and
the
world line
is
a
straight
one
hence,
for
an
arbitrary
choice of
coordinates,
a geodetic
line.
That the
point’s
world line
is
a
geodetic
line otherwise
as
well
(if
others do
not exert
gravitational
influence)
is
a
hypothesis,
albeit
one
that
suggests
itself.-
You
are
right
with
your
criticism
on page
178
(footnote).
On
close exami-
nation,
the
demand for
causality
is
merely
not
sharply
defined.[2]
The
causality
requirement can
be fulfilled to various
degrees.
It
can
only
be said
that the
general
th.
of
r[elativity]
has been
more
successful
than
classical mechanics in
satisfying
it.
It
might
be
a rewarding
task
for
an
epistemologist
to
think this
through
carefully.
Cordial
greetings, yours,
A. Einstein.
P.
S.
I
am
sending you
a new
paper
which treats
a
principal
point
of
the
gen.
th.
of
rel.[3]
315.
From
Hendrik A. Lorentz
Haarlem,
22
March
1917
Dear
Colleague,
We
were very sorry
to hear from Ehrenfest
that
recently your
health
leaves
something
to be
desired[1]
and
that
for
this
reason
you
will
not be able to
attend
the
Congress
of Dutch Scientists and
Physicians
to be held
during
the Easter
holidays.[2]
I
understand
very
well
your
wish to avoid
the
fatigue
attached
to
such
gatherings;
and
as
much
as we
would
enjoy your
participation,
it
is
much
more
important
that
you
convalesce
properly
as soon
as
possible.
Should
you
occasionally
find it
hard
to
be
easy on yourself as
precaution
would
require,
I
beg
you
to
think
of
your many
friends and
colleagues,
who
dearly
wish
to
see
you
healthy
and
vigorous.
After
the
exhausting
work of
the last
few
years, you surely
have
earned
some
rest.
Could
you
not
leave Berlin
now
for
a
time
to
seek
relaxation
in Switzerland
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