"The start of the revolution that produced the old quantum theory is moved from the end of 1900 to 1906... The preceding crisis...resulted from the difficulties in reconciling Planck's derivation with the tenets of classical physics. Planck's change in vocabulary—from "resonator" to "oscillator" and from "element" to "quantum"—is the central symptom of incommensurability. It signals the changed meaning of the quantity hv from a mental subdivision of the energy continuum to a physically separable atom of energy. That my critics continue to apply the term "energy quantum" to pre-1906 papers and lectures in which Planck consistently used "energy element" reveals something of the difficulty of reversing the gestalt switch that took place during that year and those which followed. ...Boltzmann's probabilistic derivation of the entropy of a gas ...illustrates the problem to which the concept of paradigm was a response. The derivation was not reduced to rules but instead served as a model to be applied by means of analogy. As a result, when its application was transferred from gases to radiation, Planck and Lorentz could invent different analogies with which to effect change."