  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.
