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Ludwik Fleck says that a thought collective develops a thought-style which shapes the ways of perceiving the world and thinking of the world by its members. So how could it happen that at the end of 1900 Max Planck, whose thinking was determined by classical mechanics, managed to think that energy is quantized - the idea that contradicted the principles of Newton's mechanics? The author answer is that Planck did not intend to think it. From 1880 Planck tried to reconcile the time reversibility of the laws of mechanics with the time irreversibility of the laws of thermodynamics. The irreversibility in question was expressed in 1854 by Clausius in the form of the law of the growth of entropy defined as dS = dQ/T. During 1870's Boltzmann found the statistical interpretation of entropy as S = k ln w + const. At first Planck insisted on applying mechanics in a unrestricted way. He criticized Boltzmann statistical physics and rejected atomism as leading to Loschmidt's paradox. For 20 years Planck did not manage to solve his central problem. From about 1897 Planck started to cope with the problem of irreversibility using experimental data for the energy distribution of black body radiation. When he failed again he started to apply some statistical techniques. Using the laws of classical thermodynamics and adjusting his formulae to experimental data he arrived at the formula for the mean entropy of resonators. When in October 7th 1900 Heinrich Rubens gave him improved data for the energy distribution for big values of lambdaT, Planck at once corrected his expression for the average entropy of resonators and arrived at the formula for the distribution of the energy of black body radiation that was in perfect agreement with the experimental data. Just after that, trying to provide his results with 'the real physical meaning' he tried statistical methods like as developed by Boltzmann - whom he criticized for twenty years - and in December 1900, using purely theoretical combinatorial methods, he arrived at the formula for the average entropy of resonators. It turned out that both formulae were identical if and only if the energy of resonators was complete multiplicity of hv. So Planck was thinking according to rules inherited from others and quanta appeared rather on paper then in his mind.