This book is about the influence of varying theological conceptions of contingency and necessity on two versions of the mechanical philosophy in the seventeenth century. Pierre Gassendi (1592–1655) and René Descartes (1596–1650) both believed that all natural phenomena could be explained in terms of matter and motion alone. They disagreed about the details of their mechanical accounts of the world, in particular about their theories of matter and their approaches to scientific method. This book traces their differences back to theological presuppositions they inherited from the Middle Ages. Theological ideas were transformed into philosophical and scientific ideas which led to the emergence of different styles of science in the second half of the seventeenth century.

"Her book cannot fail to leave an enduring mark on our understanding of the Scientific Revolution....a major contribution to the growing but still small literature on Gassendi; she absolutely rejects the interpretation of Gassendi as covert materialist. The book includes as well excellent brief expositions of two different mechanical philosophies, which will in themselves be of interest to historians of seventeenth century science....It will be evident from the review above that I find Osler's argument convincing. Her book brings an unexpected perspective to bear on the Scientific Revolution, such that every student of seventeenth century science will need to consider it. Even those who may disagree with Osler's thesis will find, I believe, that they cannot ignore it." Richard Westfall, The Philosophical Review

"Osler's book is a major contribution to the growing but still small literature on Gassendi....Her book brings an unexpected perspective to bear on the Scientific Revolution, such that every student of seventeenth century science will need to consider it. Even those who may disagree with Osler's thesis will find, I believe, that they cannot ignore it." The Philosophical Review

"This is a wonderful book, the culmination of twenty years of patient and painstaking scholarship." International Philosophical Quarterly