Planck’s law for black-body radiation marks the origin of quantum theory and is discussed in all
introductory (or advanced) courses on this subject. However, the question whether Planck really
implied quantisation is debated among historians of physics. We present a simplified account of this
debate which also sheds light on the issue of indistinguishability and Einstein’s light quantum
hypothesis. We suggest that the teaching of quantum mechanics could benefit from including this
material beyond the question of historical accuracy.