When two hypotheses are possible, we provisionally choose that which our minds adjudge to be simpler, on the supposition that this is the more likely… - James Hopwood Jeans

In every previous application of the quantum law, Planck's law, that the energy is h [Planck's constant] times the frequency, had been used to deduce the energy of a quantum when the frequency of the radiation was already known. In the present case the formula was used the other way; the energy of the emitted photon was known to begin with, and the formula was utilized to deduce its frequency. The frequencies calculated in this way are found to agree completely and exactly with those of the spectrum of hydrogen.

A second conspicuous landmark... is the enunciation of the fundamental law of radioactive disintegration by Rutherford and Soddy in 1903. This law was in no sense a consequence or development of Planck's theories; indeed fourteen years were to elapse before any connection was noticed between the two. The new law asserted that the atoms of radioactive substances broke up spontaneously, and not because of any particular conditions or special happenings. This seemed to involve an even more startling break with classical theory than the new laws of Planck; radioactive break-up appeared to be an effect without a cause, and suggested that the ultimate laws of nature were not even causal.

...a detailed mathematical discussion shows that whatever kind of wave-packet we select to represent the electron, the product of the two uncertainties of position and momentum can never be less than h, which is precisely what Heisenberg found...