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Fermi's way of teaching and thinking about quantum mechanics deserves special mention. His attitude was entirely pragmatic. Quantum mechanics is acceptable because its predictions agree with experiment. He once said that "the Schrödinger equation has no business agreeing so well." Nothing else counted. He devoted no time to such topics as the quantum theory of measurement. He was immune to the "Copenhagen spirit," both by temperament and by educational background. He was completely self-taught in quantum mechanics, an outsider to the Göttingen-Zürich-Copenhagen founders' circle. Fermi drew a firm line between physics and philosophy. Although he was endowed with remarkable analytic powers, Fermi often affected an aversion to abstract mathematics.
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Lee tells the story that Fermi, at some point, decided to teach group theory to his private seminar. He took out his index cards on the subject and started first to discuss Abelian groups, then Burnside's theorem, and next the notion of a group's Center. Only much later did he get to the group concept itself. Some of the students expressed confusion at this seemingly erratic approach. "Group theory is nothing but a compilation of definitions," replied the master. So he was simply following the index at the end of Hermann Weyl's book in alphabetical order.