text: Beyond question, it was his work as a theorist.  The story about the falling apple is about as apocryphal as the one about George Washington and the cherry tree.\n\nTo appreciate what Newton did, we must realize that he came after Kepler and Galileo.  Kepler, using Tycho's observational data, formulated three laws of planetary motion -- ellipses, equal areas in equal times, and the cube of the orbital radius equals the square of the period.  He did <not> explain why these laws obtained.  That was left to Newton.\n\nGalileo was a great experimenter, observer, and theorist.  His work with the inclined plane and falling objects produced g, the acceleration due to gravity ... but Galileo did not have the calculus, which explains instantaneous motion.  That was left for Newton.\n\nBecause of Kepler and Galileo, Newton had all the data he needed.  His great work -- all theoretical -- was to explain the results obtained by Kepler and Galileo.  Newton's three laws of motion -- uniform linear motion, F = ma, and conservation of momentum -- are all theoretical constructs which, when coupled with the calculus (his invention) and applied to "action at a distance" -- universal gravitation -- Kepler's and Galileo's results were derivable.\n\nHis invention of the calculus, described opaquely in the Principia Mathematica, was for use as a tool to support his work on gravitation.\n\nNewton was no slouch as an experimentalist -- sometimes.  He performed top quality leading-edge experiments with optics, backing up his results with theoretical insights.  He also devoted considerable time and energy to the pseudoscience of alchemy -- turning base metals into gold -- with lesser success.\n\nBut it was his theoretical work that established his lasting reputation.