DOC. 4 FOUNDATIONS OF THERMODYNAMICS 97
(Doc. 5),
pp.
355-357. Einstein's
way
of
intro-
ducing
his
assumption
about the
probabilities
of
state
distributions-"wir
werden anzunehmen
haben"-suggests that he
may
not
have
re-
garded
the
assumption as a putatively
true
asser-
tion about the actual behavior
of
the
systems
constituting
the
ensemble,
but
rather
as a hy-
pothesis necessary
for
the derivation
of
the
sec-
ond law in its strict classical
thermodynamic
form from statistical
premises,
which
is
exactly
how Einstein described it in his letter
to
Besso
of
22
January
1903
(see
the
editorial
note,
"Einstein
on
the Foundations
of
Statistical
Physics,"
p.
51).
For later comments
on
the
statistical
character of
the second
law,
see
Dis-
cussion/Einstein
1911,
pp.
436-443,
and
Ein-
stein 1915a,
pp.
261-262.
[18]
Here
v,
which
was
used
above to index the
members
of
a
virtual
ensemble
of
N
replicas
of
a
single system, is
used to index the
subsystems
a1, a2,
...
of
the
composite system.