Cross phenomena

representing responses of a system to external stimuli

are ubiquitous from quantum to macro scales. The Onsager theorem is often used to describe them

stating that the coefficient matrix of cross phenomena connecting the driving forces and the fluxes of internal processes is symmetric. Here we show that this matrix is intrinsically diagonal when the driving forces are chosen from the gradients of potentials that drive the fluxes of their respective conjugate molar quantities in the combined law of thermodynamics including the contributions from internal processes. Various cross phenomena are discussed in terms of the present theory.