True equilibria in closed systems and stationary equilibria in open systems show similarity, inasmuch as the system, taken as a whole and in view of its components, remains constant in both. But the physical situation is fundamentally different. Equilibria in closed systems are based on reversible processes; they are a consequence of the second principle of thermodynamics and are defined by a minimum of free energy. In open systems, by contrast, the steady state is not reversible as a whole nor in many individual reactions. A closed system must according to the second principle, eventually attain a time-independent steady state of equilibrium defined by maximum entropy and minimum free energy where the ratio between its components remains constant. An open system may attain a time-independent steady state, even though there is a continuous flow of component materials, but is more likely to exhibit characteristics of organismic systems such as dynamic equilibrium, adaptation, and self-regulation.