> "I used to take long walks on Sundays and get away from the work altogether, and at the end of those walks I would perhaps go on with my work a bit in a refreshed state of mind. And after one of these Sunday walks it occurred to me that the commutator might be the analogue of the Poisson bracket, but I didn’t know very well what a Poisson bracket was then. I had just read a bit about it and forgotten most of what I had read, and I wanted to check up on this idea, but I couldn’t do it because I didn’t have any book at home which gave Poisson brackets and all the libraries were closed. So I just had to wait impatiently until Monday morning when the libraries were open to go and check up on what Poisson brackets really were. Then I found that they did fit, but I had one impatient night of waiting."
[1] https://www.aip.org/history-programs/niels-bohr-library/oral...