IN his age of celebrity, Newton was asked how he had discovered the law of universal gravitation. “By thinking on it continually,” was the reply. No better characterization of the man can be given, not only in its delineation of a life whose central adventure lay in the world of thought rather than action, but also in its description of his mode of work. Seen from afar, Newton's intellectual life appears unimaginably rich. He embraced nothing less than the whole of natural philosophy, which he explored from several vantage points which ranged all the way from mathematical physics to alchemy. Within natural philosophy, he gave new direction to optics, mechanics, and celestial dynamics, and he invented the mathematical tool which has enabled modern science further to explore the paths he first blazed. He sought as well to plumb the mind of God and His eternal plan for the world and mankind as it was presented in the biblical prophecies. When we examine Newton's grandiose adventure minutely, it curns out to be a mixture of discrete pieces rather than a homogeneous melange. His career was episodic. What he thought on, he thought on continually, which is to say exclusively, or nearly exclusively. What seized his attention in 1664, to the virtual exclusion of everything else, was mathematics.