DOC.
343 MAY 1917
333
343. To
Moritz Schlick
[Berlin,]
21
May
1917
Esteemed
Colleague,
Time
and
again
I
look
over
your
little
book and
delight
in the
splendidly
clear
discourse.[1] I
also find
the last
section,
“Links
to
Philosophy,” superb.
If
something
did catch
my
eye
while
occupied
with
this
ruminating business,
I let
you know,
so
that
you
can
make
some
corrections
for
an even[tual] new
edition.
The
discussion
on
the
invalidity
of Euclidean
geometry
at
the
top
of
page
33
is
misleading.
It
cannot be said
that
Euclidean
geometry
is
not valid for two
systems
rotating
relative to each other.
Rather,
the
following
can
be deduced:
Assuming system
K
is
Galilean,
or
that
a
system
K
exists,
in which
the
possible
positionings
of
practically rigid
bodies
(at
least in
a
certain
region)
at rest
rel. to
K
are
governed by
Eucl.
geometry,
then the
same
surely
is
not
the
case
for
a
system
K'
rotating
relative to
K.-(In
this
proof, systems
K
and
K'
thus
play entirely
different
roles.)
From
this
it
is
deduced first
that the
existence of
a
gravitational
field
precludes
the
validity
of Euclid.
geometry (a
field
exists,
of
course,
rel. to
K').
Finally,
from
the
circumstance that,
upon
closer
examination, gravitational
fields
are
absolutely
never
absent,
it
is
deduced
further
that
in
reality
Galilean
coordinate
systems
do
not
exist at all within finite
areas,
that
therefore Eucl.
geometry
is
never
valid within finite
spaces.-[2]
The
second
point I
would
like
to
make
regards
the
concept
of
reality.[3]
Your
interpretation
compares
to
Mach’s
according
to
the
following
scheme:
Mach:
Only perceptions
are
real.
Schlick:
Perceptions
and events
(of
a
phys.
nature)
are
real.
Now,
it
seems
to
me
that
different
senses
of
the
word
“real”
are
being used,
depending
either
on
whether
perceptions
are
being
discussed
or
events
and facts
in
the
physical
sense.
When
two
different nations
engage
in
physics independently
from each
other,
they
will create
systems
that
certainly concur concerning
the
perceptions
(“ele-
ments” in Mach’s
sense).
The theoretical constructions
that the
two devise
to
connect
these “elements”
can
be
considerably
different.
Both constructions
do
not need to
agree
with each
other
either
regarding
the
“events”;
because
they
clearly belong
to
the
conceptual
constructions.
Certainly only
the
“elements”
are
real,
in
the
sense
of
“irrefutably
given
in
experience,”
but the
“events”
are
not.
If,
however,
we
denote
as
“real”
what
we
have
classified
in
the
space-and-time
scheme,
as
you
have done in
your
[General] Epistemology,[4]
then
no
doubt,
the
“events”
are
real in
the
first
place.
What
we
describe
as
“real”
in
physics
is
unquestionably
that
which
is
“spatio-
temporally classified,”
not what
is
“immediately
given.”
What
is
immediately
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