The exclusion principle was introduced in 1924, right at the end of a period of crisis for the Bohr–Sommerfeld so-called “old quantum theory”. An increasing number of spectroscopic anomalies challenged the old quantum theory between 1920 and 1924, and prompted a radical theoretical change that paved the way to quantum mechanics after 1925. Trapped in between the strictures of an inadequate old quantum theory, on the one hand, and a not-yet-developed new quantum theory on the other hand, the history of Pauli’s principle sheds light on that complex revolutionary transition around 1925. At the same time, it raises important philosophical questions about the nature of scientific principles, and on the mechanism though which an “extremely natural” empirical rule (as Pauli originally presented it) could become a fundamental building-block of physics.