In the 1960s Kuhn’s historical work turned toward the early history of quantum theory, culminating in his book Black-Body Theory and the Quantum Discontinuity. According to classical physics a particle could possess any energy in a continuous range and if it changes energy it does so in a continuous fashion, possessing at some point in time every energy between the initial and final energy states. Modern quantum theory denies both these classical principles. Energy is quantised—a particle may possess only one of a set of discrete energies. Consequently if it changes in energy from one value to the next permitted value it does so discontinuously, jumping straight from one energy to the other without taking any of the intermediate (‘forbidden’) values. In order to explain the distribution of energy within a cavity (black-body radiation), Planck used the device of dividing up the energy states into multiples of the unit or ‘quantum’ hν (where ν is the frequency of radiation and h is what subsequently became known as Planck’s constant). Planck did this in order to employ a statistical technique of Boltzmann’s whereby the range of possible continuous energies is divided into ‘cells’ of similar energies that could be treated together for mathematical purposes. Kuhn notes that Planck was puzzled that in carrying out his derivation, only by fixing the cell size at hν could he get the result he wanted—the technique should have worked for any way of dividing the cells, so long as they were small enough but not too small. This work of Planck’s was carried out in the period 1900–1, which is the date tradition has accorded to the invention of the quantum concept. However, argued Kuhn, Planck did not have in mind a genuine physical discontinuity of energies until 1908, which is after Albert Einstein and Paul Ehrenfest had themselves emphasized it in 1905–6.