Thermodynamics sets up an idealized conceptual structure that can be summarized by a formal scheme of definitions and postulates.
Thermodynamic states are amongst the fundamental or primitive objects or notions of the scheme, for which their existence is primary and definitive, rather than being derived or constructed from other concepts.
The choice is usually made on the basis of the walls and surroundings that are relevant for the thermodynamic processes that are to be considered for the system.
For Planck, the primary characteristic of a thermodynamic state of a system that consists of a single phase, in the absence of an externally imposed force field, is spatial homogeneity.
The primary or original identification of the thermodynamic state of a body of matter is by directly measurable ordinary physical quantities.
For a given body, of a given chemical constitution, when its thermodynamic state has been fully defined by its pressure and volume, then its temperature is uniquely determined.
For an idealized continuous or quasi-static process, this means that infinitesimal incremental changes in such variables are exact differentials.
The state functions satisfy certain universal constraints, expressed in the laws of thermodynamics, and they depend on the peculiarities of the materials that compose the concrete system.
Based on many observations, thermodynamics postulates that all systems that are isolated from the external environment will evolve so as to approach unique stable equilibrium states.