  Compounding the diversity of the subjects to which Newton devoted time are sharp contrasts in his work within each subject. Optics and orbital mechanics both fall under what we now call physics, and even then they were seen as tied to one another, as indicated by Descartes' first work on the subject, Le Monde, ou Traité de la lumierè. Nevertheless, two very different “Newtonian” traditions in physics arose from Newton's Opticks and Principia: from his Opticks a tradition centered on meticulous experimentation and from his Principia a tradition centered on mathematical theory. The most important element common to these two was Newton's deep commitment to having the empirical world serve not only as the ultimate arbiter, but also as the sole basis for adopting provisional theory. Throughout all of this work he displayed distrust of what was then known as the method of hypotheses – putting forward hypotheses that reach beyond all known phenomena and then testing them by deducing observable conclusions from them. Newton insisted instead on having specific phenomena decide each element of theory, with the goal of limiting the provisional aspect of theory as much as possible to the step of inductively generalizing from the specific phenomena. This stance is perhaps best summarized in his fourth Rule of Reasoning, added in the third edition of the Principia, but adopted as early as his Optical Lectures of the 1670s: