Looking at Pauli’s scientific correspondence and the way he originally introduced the principle in a letter to Alfred Landé in 1924, it was clear that it was a purely phenomenological rule to explain some puzzling phenomena in spectroscopy on which Pauli and colleagues had been working for years. Heisenberg referred to it teasingly as Pauli’s “Verbot”; and it was only with Dirac that it became known as “Pauli’s exclusion principle” in 1926. How did a phenomenological rule eventually become a scientific principle?‘

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