xlviii INTRODUCTION
TO VOLUME
8
ment
(see
section
II),
it must have been
especially gratifying
for Einstein
that
Schwarzschild,
director of
the
Astrophysical Observatory
in
Potsdam, immediately
took
a strong
interest
in the
theory. Unfortunately,
Einstein’s
correspondence
with
Schwarzschild
was
cut short
by
the latter’s
untimely
death in
May
1916. Einstein
became
a
member of
the
search committee
to
replace
Schwarzschild. In this
capac-
ity,
he asked
another
correspondent,
the
Leyden
astronomer Willem de
Sitter,
for
his
opinion
of
various candidates
(Doc. 243).
During a
visit
to the Netherlands
in
the fall
of
1916,
Einstein and De
Sitter
started
a
discussion
about,
as
they
called
it,
the
relativity
of
inertia.
In
the
course
of
the
lengthy correspondence
that
ensued,
Einstein modified the field
equations
of
1915 to allow for
a
static
spatially
closed
model
of
the universe
(Einstein
1917b
[Vol. 6,
Doc.
43]). Interestingly,
Einstein
had also sounded the theme
of
the
relativity
of
inertia in his
correspondence
with
Schwarzschild
(Doc. 181);
and Schwarzschild had
mentioned
in
passing
that
his
solution
for
the interior
of
a
fluid
sphere
contains
as a special case
what
we now
recognize as
the De Sitter solution
(Doc. 188).
This solution became the focus
of
the debate
between
Einstein and De Sitter in
1917.[23]
VII
The
cosmology paper,
Einstein 1917b
(Vol. 6,
Doc.
43),
and the discussion
of
it
in
correspondence
with De Sitter
and,
in
early
1918,
with Mie led Einstein to
rethink
the foundations
of
general relativity.
In Einstein
1918f
(Vol.
7,
Doc.
4),
he identi-
fied three
principles as forming
the
basis of
the
theory:
the
relativity principle,
the
equivalence principle,
and Mach’s
principle.
It is instructive
to
compare
these three
principles
to
their
precursors
in 1914 and 1916 when
Einstein
discussed the foun-
dations
of
the “Entwurf”
theory
and the final
theory
of
November 1915 in
lengthy
systematic expositions
of
these
theories.[24]
Such
a comparison
will
give
some
sense
of
the
theory’s bewildering conceptual development.
At
the
same
time,
it will
illustrate Einstein’s intellectual
flexibility
and his
willingness openly
to admit
error.
Einstein
justly
prided
himself
on
these
traits,
even though
he could also be
excep-
tionally
stubborn and
even though
he chose the
errors
he
was
prepared
to admit and
the
people
he admitted
them to with
some care.
Einstein’s formulation
of
the
relativity principle
in 1918 is
as
follows: “The laws
of
nature
are
statements about
spatio-temporal
coincidences
only.”
This is
a
far
cry
from what
Einstein
had envisioned in
1914,
when he
triumphantly
announced to
several
correspondents (Docs.
5
and
14)
that
the
severely
restricted
covariance
of
the “Entwurf”
theory
was
nonetheless
broad
enough
to
include
arbitrarily
acceler-
ated frames
of
reference,[25]
thus
extending
the
principle
of
relativity
to
arbitrary
motion. In
1918,
the
theory
is
generally
covariant,
but the statement
of
the
relativity