A thermodynamic minimum principle valid for photon radiation is shown to ho
ld for arbitrary geometries. It is successfully extended to neutrinos, in t
he zero mass and chemical potential case, following a parallel development
of photon and neutrino statistics. This minimum principle stems more from t
hat of Planck than that of classical Onsager-Prigogine irreversible thermod
ynamics. Its extension from bosons to fermions suggests that it may have a
still wider validity.