I was learning statistical mechanics and referred to the book "statistical thermodynamics" by Tien, C. L.; Lienhard, J. H.
With reference to Herbert Callen's book, Postulates of Macroscopic thermodynamics was given.
Callen has combined the physical information that is included in the Laws of thermodynamic into a set of four postulates.
The first Postulate is:
There exist certain states (called equilibrium states) of simple systems that macroscopically characterized completely by Internal Energy ($U$), Volume($V$), and number of moles ($N_1,N_2,\cdots N_r$) of the $r$ chemical components.
In the context of this postulate, it has been mentioned that
Suppose that an isolated system is made up of several subsystems which may or may not be isolated from one another. The problem is then that of determining the final state of this composite system when any such internal constraints as might exist (e.g. walls of any kind) are removed. The first postulate implies that such a final state will be unique but gives no way of determining it.
Why do we need these postulates and how did these postulates make our life easy in statistical thermodynamics. I think these postulates a slight rearrangement of laws of thermodynamics?
I didn't Understand How this first postulate implies that the final state of the system will be unique. and in the first place what do we mean by unique here. I think the most probable macrostate will be the final state, then from where the question of uniqueness comes?