XXth century_physics

”I used to take long walks on Sundays alone, thinking about
these problems and it was during one such walk that the idea
occurred to me that the commutator A times B minus B times A
was very similar to the Poisson bracket which one has in
classical mechanics when one formulates the equations in the
Hamiltonian form. That was an idea that I just jumped at as
soon as it occurred to me. But then I was held back by the fact
that I did not know very well what was a Poisson bracket. It was
something which I had read about in advanced books of
dynamics, but there was not really very much use for it, and
after reading about it, it had slipped our of my mind and I did
not very well remember what the situation was. It became
necessary to check whether the Poisson bracket really could be
made to correspond to the commutator and I needed to have
a precise definition of the Poisson bracket.”
Paul Dirac