The origins of this book go back to 1966, when I had the honor of giving the Wiles Lectures in the Queen's University of Belfast, sponsored by the foundation established by Mrs. Janet P. Boyd in memory of her father. This foundation is remarkable in its conception. It not only provides a lecturer on an aspect of history, but ensures that each lecture will be discussed by the Belfast historians and research students and an invited group of historians from other universities. The evening discussions, following each afternoon's lecture, were of great value in helping me to make more precise certain basic issues. I am especially grateful for having thus been able to test certain primary viewpoints in an audience of colleagues and of general historians, and to profit by the reactions of Rupert and Marie Boas Hall, John Herivel, Michael Hoskin, George Huxley, D. T. Whiteside, and W. P. D. Wightman. I am indebted to my academic host, Professor J. C. Becket, to Mrs. Janet P. Boyd, and to Vice-Chancellor and Mrs. Michael Grant for much personal kindness.

The completion of a published version of these lectures occurs a decade or so later than had been expected. This delay has been caused, in the first instance, by the consuming labor of completing the Introduction to Newton's ‘Principia' and of the editing of Newton's Principia with variant readings (undertaken in concert with Alexandre Koyre and with the assistance of Anne Whitman).