In irreversible thermodynamics (de Groot & Mazur), the entropy production rate σ = Σᵢ JᵢXᵢ ≥ 0 where Jᵢ are thermodynamic fluxes (heat, matter, charge) and Xᵢ are conjugate forces (temperature gradient, chemical potential gradient, electric field). Near equilibrium, Onsager's linear response Jᵢ = Σⱼ LᵢⱼXⱼ holds with symmetric coefficients (Lᵢⱼ = Lⱼᵢ). The minimum entropy production principle applies at steady states far from equilibrium driven by fixed boundary conditions.