This function cannot be determined from thermodynamic reasoning alone, and Wien guessed at the form that was valid at high frequency. He supposed that the average energy in high-frequency modes was suppressed by a Boltzmann-like factor: ⟨ ⟨ E f ⟩ ⟩ = e − − β β h f . This is not the expected classical energy in the mode, which is 1 / 2 β β by equipartition, but a new and unjustified assumption that fit the high-frequency data.