This chapter explores the heuristic fruitfulness of the exclusion principle in opening up new avenues of research: namely the idea of ‘coloured’ quarks and the development of quantum chromodynamics (QCD) in the 1960s to 1970s. Sections 5.1 and 5.2 reconstruct the origin of the quark theory from Gell-Mann's so-called ‘eightfold way’ for elementary particles. The experimental discovery of the Ω− particle confirmed the validity of Gell-Mann's model, but it also provided negative evidence against quarks obeying the exclusion principle. Two alternative research programmes emerged in the 1960s to deal with this piece of negative evidence (Section 5.3): the first programme (Section 5.3.1) rejected the strict validity of the exclusion principle and explored the possibility that quarks obeyed parastatistics; the second (Section 5.3.2) retained the exclusion principle and reconciled it with negative evidence by introducing a further degree of freedom for quarks (‘colour’). The Duhem–Quine thesis seems to loom on the horizon (Section 5.4): the choice as to whether questioning the principle or introducing an auxiliary assumption to reconcile it with negative evidence seems to be underdetermined by evidence. However, I shall argue that it was exactly via the development of these two rival research programmes that the exclusion principle came to be validated, and that there was a rationale for retaining the principle despite prima facie recalcitrant evidence.

Introduction

As we have seen in Chapter 4, soon after 1924 Pauli's exclusion rule was incorporated into the growing quantum mechanical framework, where its role came to be redefined and its nomological scope extended.