 Many legends concerning momentous events in history are apocryphal, but the legend of Halley’s visit to Newton in 1684 is not: it explains what prompted Newton to write his magnum opus. In August of 1684, Edmond Halley—for whom the comet is named—came to visit Newton in Cambridge in order to discover his opinion about a subject of much dispute in celestial mechanics. At this time, many in the Royal Society and elsewhere were at work on a cluster of problems that might be described as follows: how can one take Kepler’s Laws, which were then considered among the very best descriptions of the planetary orbits, and understand them in the context of dynamical or causal principles? What kind of cause would lead to planetary orbits of the kind described by Kepler? In particular, Halley asked Newton the following question: what kind of curve would a planet describe in its orbit around the Sun if it were acted upon by an attractive force that was inversely proportional to the square of its distance from the Sun? Newton immediately replied that the curve would be an ellipse (rather than, say, a  circle).[17]  Halley was amazed that Newton had the answer at the ready. But Newton also said that he had mislaid the paper on which the relevant calculations had been made, so Halley left empty handed (whether there was any such paper is a subject of dispute). But he would not be disappointed for long. In November of that year, Newton sent Halley a nine-page paper, entitled De Motu (on motion), that presented the sought-after demonstration, along with several other advances in celestial mechanics. Halley was delighted, and immediately returned to Cambridge for further discussion. It was these events that precipitated the many drafts of De Motu that eventually became Principia mathematica by 1686. Several aspects of the Principia have been central to philosophical discussions since its first publication, including Newton’s novel methodology in the book, his conception of space and time, and his attitude toward the dominant orientation within natural philosophy in his day, the so-called mechanical philosophy, which had important methodological consequences.