Sir Isaac Newton, in the year 1687, published his research work in Philosophiae Naturalis Principia Mathematica. This work described universal gravitation and the three laws of motion. These theories, since more than three centuries now, have been dominantly used, in most scientific calculations and applications.
Have you ever wondered how he first got the thought of gravity or motion? How did he work out those concepts in simple mathematical formulas? What principles and assumptions did he use and how? How many times did he test and revise his formulas, principles and assumptions after practical application? What different did he do?