The first term on the right hand side of the equation presents a stream of thermal energy into the system; the last term—a stream of energy into the system coming with the stream of particles of substances formula_12 that can be positive or negative, formula_13 is chemical potential of substance formula_14. The middle term in (1) depicts energy dissipation (entropy production) due to the relaxation of internal variables formula_15. In the case of chemically reacting substances, which was investigated by Prigogine, the internal variables appear to be measures of incompleteness of chemical reactions, that is measures of how much the considered system with chemical reactions is out of equilibrium. The theory can be generalized, to consider any deviation from the equilibrium state as an internal variable, so that we consider the set of internal variables formula_15 in equation (1) to consist of the quantities defining not only degrees of completeness of all chemical reactions occurring in the system, but also the structure of the system, gradients of temperature, difference of concentrations of substances and so on.