Planck immediately set about attempting to understand the significance of his formula for black-body radiation. He began by using Boltzmann's procedure in statistical mechanics, an approach he had previously rejected, but then adopted empirically a definition of the entropy of the oscillators which introduced the concept of quantisation. HIs derivation was not understood by his contemporaries, including Einstein, because of the lack of a theoretical motivation for the definition of entropy. Despite a major effort to understand his formula, Planck found no classical solution to the meaning of h, Planck's constant.