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