Thermodynamic equilibrium is a primitive notion of the theory of thermodynamics. According to P.M. Morse : "It should be emphasized that the fact that there are thermodynamic states,..., and the fact that there are thermodynamic variables which are uniquely specified by the equilibrium state... are not conclusions deduced logically from some philosophical first principles. They are conclusions ineluctably drawn from more than two centuries of experiments." This means that thermodynamic equilibrium is not to be defined solely in terms of other theoretical concepts of thermodynamics. M. Bailyn proposes a fundamental law of thermodynamics that defines and postulates the existence of states of thermodynamic equilibrium.