问题不在于[[引力]]是否存在，而在于它是否能从地球延伸到如此远，还能够成为保持月球在轨道运行的力。牛顿发现，如果让该力随距离的平方反比而减少，所计算出的月球轨道周期能与真实情况非常好地吻合。他猜想同样的力也导致了其他的轨道运动，并因此将之命名为“万有引力”。

同時代的作家[[威廉·斯蒂克利]]牧師在他的《[[艾薩克·牛頓爵士生平回憶錄]]》中记录了1726年4月15日他在[[肯辛顿]]与牛顿的一次谈话<ref>[http://www.trivia-library.com/b/sir-isaac-newton-theory-of-gravity-and-the-falling-apple.htm Trivia on Sir Isaac Newton's Theory of Gravity and the Falling Apple |Trivia Library]</ref>，在该次谈话中，牛顿回忆了“从前，引力的概念进入了他的脑海。在他正在沉思时，苹果的下落引起了他的思考。为什么苹果总会垂直地落在地上，他心中想到。为什么就不能走侧面或者向上升，却永远地朝向地球的中心。”<ref>“when formerly, the notion of gravitation came into his mind. It was occasioned by the fall of an apple, as he sat in contemplative mood. Why should that apple always descend perpendicularly to the ground, thought he to himself. Why should it not go sideways or upwards, but constantly to the earth's centre.”</ref> 相似的说法还出现在伏尔泰的著述《Essay on Epic Poetry》（1727）中：“艾萨克·牛顿爵士在他的花园里散步，首次想到了他的引力体系，接着便看见一颗苹果从树上掉下。”<ref>“Sir Isaac Newton walking in his gardens, had the first thought of his system of gravitation, upon seeing an apple falling from a tree.”</ref>

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{{科學哲學}}

{{啟蒙時代}}

{{Scientists whose names are used in physical constants}}

{{卢卡斯数学教授}}

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