The consequence of the principle of microscopic reversibility used for deriving the reciprocal relations in the linear phenomenological coefficients is derived from the classical hypothesis of the separability of individual processes. The application of the separability principle to nonlinear systems is illustrated with an example, and its ability to suggest microscopic mechanisms is demonstrated. The nature of independence among the processes is discussed in more detail than has been done before, and the number of unknowns, conditions, and the minimum number of experimental coefficients are calculated in the process of finding the independent fluxes and forces. The inapplicability of the principle of minimum entropy production to nonlinear systems is confirmed, and a new dissipation function is found which is a minimum with respect to variations in all the unprescribed forces in a steady state. Such a function is found to be the integral of part of the change of the rate of entropy production with respect to the forces as discussed by Prigogine. The integrability of such a change is due to the classical principle of separability. Since this function can only decrease with time in all natural processes, it is given the name of thermokinetic potential. A possible separation between phonon drag and the energy of transport, and also that between the charge of transfer and the electron wind effect, by measuring higher‐order phenomenological coefficients is also discussed.