‘In spite of its high-sounding name and its successful solutions of numerous problems in atomic physics, quantum theory, and especially the quantum theory of polyelectron systems, prior to 1925, was, from the methodological point of view, a lamentable hodgepodge of hypotheses, principles, theorems and computational recipes rather than a logical consistent theory. Every single quantum-theoretic problem had to be solved first in terms of classical physics; its classical solution had then to pass through the mysterious sieve of the quantum conditions or, as it happened in the majority of cases, the classical solution had to be translated into the language of quanta in conformance with the correspondence principle. Usually, the process of finding the ‘correct solution’ was a matter of skillful guessing and intuition, rather than of deductive or systematic reasoning.’

‘This state of affairs was well characterised by Sir William Bragg when he said that physicists are using on Mondays, Wednesdays, and Fridays the classical theory and on Tuesdays, Thursdays and Saturdays the quantum theory of radiation.’