Understanding the physical world was one of Leibniz's central interests. Earlier chapters of this Companion have explored aspects of Leibniz's metaphysics as they developed, his account of substance in general, and corporeal substance in particular. But Leibniz's interest in the physical world did not stop with metaphysics. Among his writings are numerous letters, notes, essays, and more extended treatments of questions in physics that show his deep engagement with the science of his day. Leibniz was one of the most important physicists of the late seventeenth century; other than Sir Isaac Newton, there is probably no other physicist of his generation who contributed more to the new mathematical physics. Thus, to understand the history of science in this crucial period, we must understand Leibniz's thoughts on physics. For Leibniz, as for many of his contemporaries, there was no clear boundary between philosophy and physics; understanding the world was at issue, and often those involved in what we would call philosophical projects to understand the world were also deeply involved in what we would consider scientific projects as well. Descartes, for example, the great philosopher of the earlier seventeenth century, made important contributions in his writings to mathematics and physics, as did Bacon, Hobbes, Gassendi, and later, Berkeley. Leibniz's physics and his philosophy were deeply intertwined; to try to study the one without the other is to get only a partial and inadequate picture of his thought.