The ‘new experimentalists’ are not so new today, about a quarter century after Ian Hacking's Representing and Intervening (Cambridge, 1983) drew philosophers' attention to the roles of experiment in the validation of scientific knowledge. One of the most insistent of the group, Allan Franklin, pleasingly presents his views for a broad audience in a new book, No Easy Answers. That experiment remains an important philosophical topic for a new generation is nicely illustrated by Michela Massimi's careful, concise study of a major quantum mechanical principle, the ‘exclusion principle’ of Wolfgang Pauli.

The thesis of Franklin's book is that ‘science is a reasonable enterprise that gives us knowledge of the natural world, based on valid experimental evidence and on reasoned and critical discussion’ (p. 227). Were this a book of a different sort, it might be appropriate to note that in this statement each noun and adjective, as well as the one transitive verb, is sufficiently problematic that a rigorous philosophical analysis ought to pin down the concepts being used. But Franklin's book is not meant to offer such an analysis so much as an ‘accurate picture’ of experimental science, mostly drawing upon twentieth-century physics, and aimed at general readers and academics without training in the natural sciences. Franklin lays out thirteen chapters' worth of case studies to support his view that scientific consensus is based on the critical use of epistemologically appropriate strategies to evaluate experimental evidence. Each case study is a kind of evidential history regarding a phenomenon or theoretical claim, and details the gradual uncovering of reasons supporting the eventual consensus position. Franklin does not shy away from presenting the uncertainties surrounding the position, or the occasional bad behaviour en route, or the judgements that are subsequently found to be mistaken. But neither does he hesitate to affirm categorically the outcomes of some of these histories. Most end with a blunt assertion, such as ‘There is no fifth force’ (p. 78).

Almost all of the case studies will be familiar to those who have read a great deal of Franklin's previous work, where you can find them discussed in greater detail. He has chapters on the fifth force, the 17-keV neutrino, the discovery of parity violation, β-decay, the controversy over Weber's high-flux gravity wave claim, and Millikan's selective use of data in the oil-drop experiment. Chapters on the Meselson-Stahl experiment testing competing models of DNA replication (the only non-physics case) and on Blas Cabrera's search for magnetic monopoles are new material for Franklin, but short. He does not propose to break new scholarly ground here. Still, the case studies are rich enough to convey the difficulty and uncertainty that characterizes experimental science, and Franklin's introduction frames them nicely with his characterization of seven strategies for validating experimental results. Of the case-study chapters, the most compelling, perhaps, are the last two, in which Franklin recapitulates persuasively his past disputes with Harry Collins over the experimenters' regress and with Andrew Pickering over the basis for retaining the Salam-Weinberg electroweak theory in the face of a possible absence of atomic parity violations.

Where Franklin is interested in defending the reasonableness of scientific inquiry via a kind of ‘working epistemology’ of experimental strategies, Michela Massimi seeks in her marvellous book a more systematic answer to a more specific question: ‘under what conditions are we justified in regarding an empirical and contingent rule as a scientific principle?’ (p. 2). Massimi takes as an example Pauli's exclusion principle and traces its history from an empirical rule of atomic spectroscopy to a principle that constrains theory construction and engendered rival research programmes for detailing nucleonic structure. The result is a subtle, thought-provoking book that exemplifies the good that can come of putting careful history of science to sound philosophical use.

Massimi places the history of the exclusion principle in a neo-Kantian philosophical framework that draws upon Cassirer and on Gerd Buchdahl's interpretation of Kant. In the view Massimi advocates, the exclusion principle does not constitute the theoretical framework in which it appears, but rather plays a systematizing role. Furthermore, it is precisely that systematizing role that is responsible for the nomological status of the principle. The motivations and implications of this view go beyond what can be treated here, but one advantage is that it takes a significant step away from treating the exclusion principle either as a convention or as an analytic truth. Subsequent chapters weave together historical and philosophical discussions with great dexterity. While the prehistory of the exclusion principle in the Bohr-Sommerfeld theory and problems arising from atomic spectroscopy is not exactly a new story, Massimi makes good use of it in preparing the ground for Pauli's suggestion, first, of the Zweideutigkeit (two-foldedness) of the electron, and later of the exclusion ‘rule’. She then extends her Kantian reading of the history by offering a Kantian gloss on the revolutionary transition from the old quantum theory to the new quantum theory, emphasizing the incorporation of the exclusion rule and the spin of the electron. Here her target is Kuhn's denial of translatability between scientific lexicons that are separated by such revolutionary transformations. Massimi argues that if taxonomies are treated as supporting systematic unity as a regulative principle rather than as objectively grounded, untranslatability does not follow. Revolutionary transitions can then be recognized as such without having to take on incommensurability.

The final two chapters trace the path by which the exclusion rule became the exclusion principle, culminating in the proof of the spin-statistics theorem, and responses to the possible violation of the exclusion principle by quarks. The latter chapter relates an especially fascinating episode, less well digested by historians than the developments in quantum electrodynamics related in the former. The challenge to the exclusion principle from quark theory appeared with the discovery of the Ω⁻, consisting of three strange quarks with parallel spin-½. As Massimi relates, two rival research programmes emerged. The parafermion theory restricted the validity of the exclusion principle, postulating that quarks were governed by statistics ‘intermediate’ between Fermi–Dirac and Bose–Einstein. The coloured quark theory, by contrast, retained the full validity of the exclusion principle but added a three-valued ‘colour charge’ as a property of quarks, such that in a given baryon, for example, each of the constituent quarks would have a different colour. Massimi deploys the history of the experimental vindication of coloured quarks in an intriguing discussion of the ‘epistemological holism’ emphasized by Quine and (differently) by Duhem. She argues that the case illustrates that epistemological holism does not automatically pose a ‘threat to the rationality of theory choice’. On the contrary, in the case at hand, epistemological holism ‘allows pieces of negative evidence within the parastatistics programme to play a positive validating role for Pauli's principle’ and ‘can convey and channel a wide-ranging array of experimental evidence to support the new auxiliary assumption of colour’ (p. 175).

The upshot is a book that makes a significant contribution to the history and philosophy of physics and offers fresh perspectives on long-standing philosophical problems regarding science. Like Franklin's book, Massimi's shows the new-experimentalist tradition to be as resourceful as ever in exploring, and defending, scientific reason.