Full Idea
Following Kepler, Newton assumed a law of universal gravitation, thus reclassifying free fall as a violent motion and, with his First Law, fixing horizontal motion in the absence of constraints as natural
Gist of Idea
Newton reclassified vertical motion as violent, and unconstrained horizontal motion as natural
Source
report of Isaac Newton (Principia Mathematica [1687]) by Rom Harré - Laws of Nature 1
Book Reference
Harré,Rom: 'Laws of Nature' [Duckworth 1993], p.26
A Reaction
This is in opposition to the Aristotelian view, where the downward motion of physical objects is their natural motion.