The form of the Stefan–Boltzmann law that includes emissivity is applicable to all matter, provided that matter is in a state of local thermodynamic equilibrium (LTE) so that its temperature is well-defined. (This is a trivial conclusion, since the emissivity, ε ε , is defined to be the quantity that makes this equation valid. What is non-trivial is the proposition that ε ε ≤ ≤ 1 , which is a consequence of Kirchhoff's law of thermal radiation.)