Newton's laws of motion Newton in his "Philosophiæ Naturalis Principia Mathematica" ("Mathematical Principles of Natural Philosophy"), first published in 1687. Newton used them to explain and investigate the motion of many physical objects and systems. For example, in the third volume of the text, Newton showed that these laws of motion, combined with his law of universal gravitation, explained Kepler's laws of planetary motion. A fourth law is often also described in the bibliography, which states that forces add up like vectors, that is, that forces obey the principle of superposition. Newton's laws are applied to objects which are idealised as single point masses,
Clarke's three laws into any typeface and size from Albertus Extra Bold to Zurich Calligraphic", referring to his memory of "seeing and hearing Linotype machines which slowly converted ‘molten lead into front pages that required two men to lift them’". A fourth law has been proposed for the canon, despite Clarke's declared intention of stopping at three laws. Geoff Holder quotes: "For every expert, there is an equal and opposite expert," which is part of American economist Thomas Sowell's "For every expert, there is an equal and opposite expert, but for every fact there is not necessarily an equal and opposite fact", from