"Experience has decided for the second alternative. But that the decision should come so soon... was due not to the examination of the law of distribution of the energy of heat radiation, still less to my special deduction of this law, but to the steady progress of the work of those investigators who have applied the concept of the quantum of action to their researches."
January 1, 1970
https://en.wikiquote.org/wiki/The_Origin_and_Development_of_the_Quantum_Theory