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.