# Thermodynamic potentials and approach to equilibrium

I have seen some material which tries to show that when approaching equilibrium upon different constraints, the changes in some thermodynamic potential is always positive or negative (for example, that Helmoltz free energy decreases for constant temperature and volume).

I cannot make sense of these arguments, however, because the quantities these thermodynamic potentials depend on are only defined at equilibrium. I think, that what one should show is that if there are several equilibria, the one which minimizes/maximizes the appropriate potential is the one which maximizes entropy. Is this the right way to think about thermodynamic potentials?