The Risk of Discovery
I'm not sure that those lines of study were actually considered separate at the time. A century earlier than Newton's time, math and geometry were considered subfields of astronomy; and astronomy was considered the observation of the realm of the divine. Not only was there not a divide between physics and theology, people actually thought that math was a tool for studying theology (one of Copernicus' sources of income was computing astrology charts for royalty).I say this to reinforce Paul's statement, "But that's because we know how things turned out." "How things turned out" includes reclassification of what he was working out as belonging to different fields. I suspect that at the time, he didn't consider himself to be moving from one field to another but rather to trying to build upon his previous work.