This book studies Locke's views on the content and method of natural philosophy. Focusing on his Essay concerning Human Understanding, but also drawing extensively from Locke's other writings and manuscript remains, it argues that Locke was an advocate of the Experimental Philosophy: the new approach to natural philosophy championed by Robert Boyle and the early Royal Society. On the question of method, this study demonstrates how Locke's pessimism about the prospects for a demonstrative science of nature led him, in the Essay, to promote Francis Bacon's method of natural history, and to downplay the value of hypotheses and analogical reasoning in science. Yet, it is argued, Locke never abandoned the ideal of a demonstrative natural philosophy, for he believed that, if we could discover the primary qualities of the tiny corpuscles that constitute material bodies, we could then establish a kind of corpuscular metric that would allow us a genuine science of nature. It was only after the publication of the Essay, however, that Locke came to realize that Newton's Principia provided a different model for the role of demonstrative reasoning in science, a model based on principles established by observation. This led Locke to make significant revisions to his views in the 1690s. On the content of Locke's natural philosophy, this book argues that, even though Locke adhered to the Experimental Philosophy he, was not averse to speculation about the corpuscular nature of matter. It takes us into new terrain and new interpretations of Locke's thought through an exploration of his mercurialist transmutational chymistry, his theory of generation by seminal principles, and his conventionalism about species.