The underlying physical laws necessary for the mathematical theory of a larger part of physics and the whole of chemistry are thus completely known. - Paul Dirac

I have the best of reasons for being an admirer of Werner Heisenberg. He and I were young research students at the same time, about the same age, working on the same problem. Heisenberg succeeded where I failed. There was a large mass of spectroscopic data accumulated at that time and Heisenberg found out the proper way of handling it. In doing so he started the golden age in theoretical physics, and for a few years after that it was easy for any second rate student to do first rate work.

The mathematician plays a game in which he himself invents the rules while the physicist plays a game in which the rules are provided by nature, but as time goes on it becomes increasingly evident that the rules which the mathematician finds interesting are the same as those which nature has chosen

In the absence of experimental evidence, basic beliefs of theoretical physicists may initially have almost a religious flavor, guided by faith and aesthetics. Fortunately unlike religion, these beliefs soon face the hard test of experiment.