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.