312
DOCS.
320,
321
APRIL
1917
320. To
Moritz Schlick
[Berlin,]
Sunday.
[1
April
1917]
Dear
Colleague,
Many
thanks
for
kindly sending
the
separates.[1]
Your
splendid
work
has al-
ready helped
a
good many people
come
to
understand the
theory,
as
I
have been
convinced.
I
approve
of
the
small
change you
are planning.[2]
I
shall be
very
pleased
if
you
call
on me again
sometime. Then
we can
also discuss
the
problem
of
the constitution
of
space.
I
recommend
to
you my
old
acquaintance Hopf,
a
capable physicist
who
is
also
employed
in
physics
in
Adlershof.[3]
Best
regards, yours,
A. Einstein.
321. From Willem de Sitter
Doorn,
Dennenoord
Sanatorium,
1
April
1917
Dear friend
Einstein,
Your
letter
pleased
me
very
much because from it
I
can
gather
that
you
are
feeling
much better.[1] As for
me,
it
is
still
the
same;
I
am
sitting-or rather
lying-here
now
in
this sanatorium
and
am slowly
recovering.
It
is not
a
serious
matter at
all, just
boring.[2]
I
understand
your position
now
fully.
Mine
was
different.[3]
[When
I
say
“I,”
you
must
not
misunderstand
me;
I
personally
take the
position
that
all
extrap-
olation
is
uncertain-it
can
be useful
at
times to
get
a
better
overview of
the
picture
we
make
of
the universe-but
most
of
the
time
it
is
dangerous,
because
it
leads
us
to
assume
that
we
have solved
a
puzzle,
when
we
have
just
clothed
it in
other
words.][4]
But
in
working
out
this
four-dimensional
world,
which
you
call
mine,
I
had
only
taken
the
position
of
those
(among
whom
I
believe Ehren-
fest
belongs)
who
require
that
the
boundary
values for guv
be
invariant,
that
is,
that the
world
as a
whole
can
perform
random motions
without
us (within
the
world)
being
able
to
observe it.
[These
“motions”
are
naturally
not
physical,
conceivable
phenomena, only
a
pseudo-physical
name
for
a purely
mathematical
concept.]
From
the
point
of view of
this mathematical
relativity condition,
“my”
four-dimensional
world
is perhaps
better than
your
three-dimensional
one.[5]
Your
condition,
the material
relativity requirement,
will
obviously
just
be satisfied
by
the
three-dimensional,
finite
world.[6] May
I
cite
this material
relativity
require-
ment from
your
letters
in
a
postscript
to
the
communication I
am
submitting
to
the Amsterdam
Academy?[7]