Is the statement of "the fundamental assumption of thermal physics" given by Kittel incorrect?

Question

I have just started re-reading Thermal Physics, by Kittel and Kroemer. They state the fundamental assumption of thermal physics as:

...a closed system is equally likely to be in any of the quantum states accessible to it. All accessible quantum states are assumed to be equally probable - there is no reason to prefer some accessible states over other accessible states.

I previously thought I understood the fundamental assumption, but apparently I do not. For an isolated system (where the number of particles and total internal energy are constant), this makes sense, but how can it be true for a closed system?

I think the issue is that the author is defining what is meant by a "closed system" differently to every other textbook I've read. Indeed, he goes on to write:

A closed system will have constant energy, a constant number of particles....

I always thought that such a system was thermally isolated, and that a closed system could have variable energy but has constant particles.

Is Kittel defining the concept of a closed system to be different to other authors, or am I simply misunderstanding something vital?

Answer 1

Well, Arnold Sommerfeld once said;

Thermodynamics is a funny subject. The first time you go through it, you don't understand it at all. The second time you go through it, you think you understand it, except for one or two small points. The third time you go through it, you know you don't understand it, but by that time you are so used to it, so it doesn't bother you any more.

But I do not think that is the issue here.

You are right in that Kittel is not using the generally accepted modern terminology but luckily he explains what he means by a closed system. Three different systems in thermodynamics are; open systems in which the system can exchange matter an energy with environment, closed systems in which energy can be exchanged with environment but not matter and isolated systems in which neither energy nor matter can be exchanged.