The first correct statement of the law of conservation of momentum was by English mathematician John Wallis in his 1670 work, "Mechanica sive De Motu, Tractatus Geometricus": "the initial state of the body, either of rest or of motion, will persist" and "If the force is greater than the resistance, motion will result". Wallis used "momentum" for quantity of motion, and "vis" for force. Newton's "Philosophiæ Naturalis Principia Mathematica", when it was first published in 1687, showed a similar casting around for words to use for the mathematical momentum. His Definition II defines "quantitas motus", "quantity of motion", as "arising from the velocity and quantity of matter conjointly", which identifies it as momentum. Thus when in Law II he refers to "mutatio motus", "change of motion", being proportional to the force impressed, he is generally taken to mean momentum and not motion. It remained only to assign a standard term to the quantity of motion. The first use of "momentum" in its proper mathematical sense is not clear but by the time of Jennings's "Miscellanea" in 1721, five years before the final edition of Newton's "Principia Mathematica", momentum or "quantity of motion" was being defined for students as "a rectangle", the product of and , where is "quantity of material" and is "velocity", .