 Philosophers have long known about the aspects of Newton’s work that are salient for understanding debates in the early modern period. For instance, no history of debates about the ontology of space and time would exclude a discussion of Newton’s famous conception of “absolute” space (see below). Similarly, any discussion of the role of hypotheses in philosophical reasoning would mention Newton prominently. These aspects of Newton’s work continue to be significant in contemporary scholarship, but the scope of discussions of Newton has greatly expanded, encompassing the whole of his intellectual life. This is especially evident in discussions of Newton’s earliest published work, which was in the field of optics. In at least three relevant respects, Newton’s early work in optics, which was published in the Philosophical Transactions of the Royal Society beginning in 1672, set the stage for the principal themes of his long career in natural philosophy (he remained active well into his seventies). Firstly, Newton’s letter to the Society’s secretary, Henry Oldenburg, often called the “New theory about light and colors”, generated an immediate, extensive, and protracted debate that eventually involved important philosophers such as Robert Hooke in Britain and Christiaan Huygens, G.W. Leibniz and Ignatius Pardies on the Continent (the beginning of the very long title of the paper is: “A letter of Mr. Isaac Newton, Mathematick Professor in the University of Cambridge, containing his New Theory about Light and Colors”). Newton consistently regarded these figures not merely as disagreeing with his views, but as misinterpreting them. This experience helped to shape Newton’s famous and lifelong aversion to intellectual controversy, a feature of his personality that he often mentioned in letters, and one that he would never outgrow. Secondly, because Newton regarded himself as having been deeply misinterpreted by his critics, he had recourse to meta-level or methodological discussions of the practice of optics and of the kinds of knowledge that philosophers can obtain when engaging in experiments with light. The novelty and power of Newton’s work in the Principia years later would eventually generate similar controversies that led him to analogous kinds of methodological discussions of his experimental practice within natural philosophy and of the kinds of knowledge that one can obtain in that field using either experimental or mathematical techniques. From our point of view, Newton’s science was unusually philosophical for these reasons. Thirdly and finally, in his earliest optical work Newton began to formulate a distinction that would remain salient throughout his long intellectual career, contending that a philosopher must distinguish between a conclusion or claim about some feature of nature that is derived from experimental or observational evidence, and a conclusion or claim that is a mere “hypothesis”, a kind of speculation about nature that is not, or not yet anyway, so derived. Newton’s much later proclamation in the second edition of the Principia (1713), “Hypotheses non fingo”, or “I feign no hypotheses”, would infuriate his critics just as much as it would prod his followers into making the pronouncement a central component of a newly emerging Newtonian method (see below for details).