I've only examined the book in a very cursory (e.g., turning pages, looking at diagrams, etc.) way, so I don't have anything profound to say. Newton's use of geometry to "prove" many mathematical properties leading to calculus is very interesting. Today, these concepts can be explained in terms of point set topology, for example. I'm amazed at how Newton was able to use the only mathematical tools available to him at the time to develop his "version" of calculus (Leibniz also).