Generally historians of science studied Planck's theory of black body radiation (1900–1906) with focus of their analysis in the question whether resonator energy is continuous or discontinuous. Following an alternative historiographical approach, we consider this question incorrect and secondary and we think that the central point for the comprehension of Planck's theoretical work consists in his evolutive thought about irreversibility.The main steps of this evolution are essentially the hypothesis of natural radiation, the elementary disorder principle, the combinatorial entropy and the phaseplane subdivision: focusing our attention on this conceptual development it is possible to understand how this theory is able to highlight some quantistic aspects without strict incompatibility with classical physics, particularly the appearance of energy elements, phase-space elementary regions and microscopical indetermination.