---P. A. M. Dirac
The Development of Quantum Theory (J. Robert Oppenheimer Memorial Prize Acceptance Speech), Gordon and Breach Publishers, New York, 1971, pp. 20-24.
The quote used an an epigraph on p. 2 in Operator Commutation Relations by Palle E. T. Jorgensen and Robert T. Moore, D. Reidel, Dordrecht / Boston / Lancaster, 1984, is an abridgement of the above passage.
A longer excerpt (with the curious substitution of "commutation" for "noncommutation" where it first appears) is presented as an epigraph in the announcement of a Program on Noncommutative Algebra at the Mathematical Sciences Research Institute.
Back when I thought about what to put into Operator Commutation Relations, I relied a lot on The Historical Development of Quantum Theory by Jagdish Mehra and Helmut Rechenberg, Springer-Verlag, New York, 1982-- . I was also impressed by how well researched this lovely book-set is. The two authors, Mehra and Rechenberg, did long interviews over the span of time when they worked on their book set. They had known Bohr, and they had many meetings with Heisenberg, Pauli, Dirac, and many more, and as I remember, much material in the book set results directly from these interviews. It is good that the two co-authors had the interviews, as these giants in quantum theory passed on shortly after the book-set was completed. I think the book set is a treasure of information on quantum theory (especially the mathematical part of it) and on the architects of the theory.
Late in life, Dirac would always tell the physicists at conferences to look to the math for clues to the deep questions in physics, and he liked to use his (Dirac) equation for the electron as an example, stressing that he was led to it by paying attention to the beauty of the math, more than to the physics experiments. He was alive when I was working on Operator Commutation Relations, and I talked to him a few times. He told me that he was happy to be quoted. He used to visit his son Gabriel Dirac (graph theory) who was my colleague in Aarhus.
Some interesting links for Dirac:
Dirac [in the MacTutor History of Mathematics archive at the University of St Andrews]
Quotations by Dirac [in the same archive]
The Nobel Prize in Physics 1933
Roundy interviews Professor Dirac
to Quotes selected by Palle Jorgensen