Review of the hardback: '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?' … 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's book is a significant contribution to the history and philosophy of physics and offers fresh perspectives on long-standing philosophical problems regarding science.' Kent Staley, British Journal for the History of Science