William Stukeley permite-nos apreciar o diáfano pensamento de Newton:
"After dinner, the weather being warm, we went into the garden and drank thea under the shade of some appletrees; only he, and myself. Amidst other discourse, he told me, he was just in the same situation, as when formerly, the notion of gravitation came into his mind. Why should that apple always descend perpendicularly to the ground, thought he to himself; occasioned by the fall of an apple, as he sat in a contemplative mood. Why should it not go sideways, or upwards? But constantly to the earth’s center? Assuredly the reason is, that the earth draws it. There must be a drawing power in matter. And the sum of the drawing power in the matter of the earth must be in the earth’s center, not in any side of the earth. Therefore does this apple fall perpendicularly or toward the center. If matter thus draws matter, it must be in proportion of its quantity. Therefore the apple draws the earth, as well as the earth draws the apple .
Thus by degrees, he began to apply this property of gravitation to the motion of the earth, and of the heavenly bodys: to consider their distances, their magnitudes, their periodical revolutions: to find out this property, conjointly with a progressive motion impressed on them in the beginning, perfectly solved their circular curves ... "
A queda de uma maçã dá início a uma linha de pensamento, que terminou com a publicação dos Principia Mathematica em 1687.
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