Few episodes in the history of modern science have been debated more than Max Planck’s creation of quantum theory in 1900. In particular, disagreement has grown about whether Planck’s theory represented a complete break with classical physics (Martin Klein) or a coherent extension of it (Thomas Kuhn). A clarification of this issue can be achieved if we look at the different traditions cooperating in Planck’s theory. Between 1899 and 1900 Planck combined specific views of radiation with his idiosyncratic take on thermodynamics to construct an innovative picture of irreversibility. During the process, he was forced to modify a substantial part of the theory as a reaction to criticisms by Ludwig Boltzmann. Planck’s subscription to certain traditions in radiation theory facilitated the integration of resources coming from kinetic theory and eventually from facilitated the integration of resources coming from kinetic theory and eventually from statistical mechanics.
By examining this episode I want to make two points. First, the quantum emerged in December 1900 as the result of a novel usage of well-established techniques and formal procedures whose roots can be traced back to the traditions of nineteenth century physics. Planck’s selection and reconfiguration of the conceptual and formal resources of these traditions constituted the essential part of his radiation theory pre- and post-1900. This reconstruction brings me to the conclusion that he did not perceive his result as a break with the previous physics. Second, discontinuity with classical physics emerged only slowly, as the outcome of a tedious work of comparison between Planck’s odd statistical procedure and traditional statistical mechanics carried out by Einstein, Ehrenfest, Jeans, Rayleigh, and Lorentz.