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A few know that Newton never claimed that the apple hit him; a few more pedants would quibble over whether 'discover' might be a better word than 'invent'. But what is perhaps most surprising is that the entire tale may have been invented as a piece of self-promotion by Newton comparatively late in life.
The most famous story about Newton comes in four versions, each of which differs slightly in tone and in the supposed effect on Newton's thought.
All versions are set in the orchard of the Manor at Woolsthorpe, during Newton's enforced sabbatical during the years that the Plague closed Cambridge, and all can be traced back directly to Newton himself.
The Stukeley Version
William Stukeley, author of Memoirs of Sir Isaac Newton's Life, claims to have had the tale direct from Newton's mouth, in the year before the great man's death.
the weather being warm, we went into the garden and drank tea, under the shade of some appletrees [...] he told me, he was in just the same situation, as when formerly, the notion of gravitation came into his mind. It was occasion'd by the fall of an apple, as he sat in a 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?
- William Stukeley
The Voltaire Version
Much of the legend of Newton comes from the writings of Voltaire, and the importance of the 'apple story' is no exception. Voltaire tells in Éléments de la Philosophie de Newton that he heard the story from Catherine Barton, Newton's niece, who presumably heard it related by Newton.
One day in the year 1666, Newton, having returned to the country and seeing the fruits of a tree fall, fell, according to what his niece, Mrs Conduitt1, has told me, into a deep meditation about the cause that thus attracts bodies in the line which, if produced, would pass nearly through the centre of the Earth.
The Conduitt Version
John Conduitt - husband of Voltaire's source and wannabe biographer of Newton - collected another version direct from the horse's mouth.
Whilst he was pensively meandering in the garden it came into his thought that the power of gravity (which brought an apple from the tree to the ground) was not limited to a certain distance from the earth but that this power must extend much further than was usually thought. Why not as high as the moon said he to himself & if so that must influence her motion...
- John Conduitt
The Pemberton Version
Henry Pemberton also knew Newton personally; he was the editor of the third edition of the Principia, the last published during Newton's lifetime.
The first thoughts, which gave rise to his Principia, he had, when he returned from Cambridge in 1666 on account of the plague. As he sat alone in the garden, he fell into a speculation on the power of gravity.
- Henry Pemberton
The Prosaic Truth...?
All these stories stem from Newton, and all serve to show the instantaneous, blinding flash of insight that changed the course of science, the semi-divine inspiration that could not have been achieved by anyone else. They also, of course, carry religious overtones, the apple2 that tempted Eve to disobey God; the metaphor for the rise of atheistic science is, to many, irresistible.
But Newton's own notes tell a different story. There is no moment, in 1666 or any other year, where a Theory of Gravity occurred to him in a flash. If any inspiration came to him in that garden, it was a beginning only. Newton would continue to puzzle over gravity for two decades; much of his earliest writings on the subject are in Latin, a language he did not adopt until 1667 at the earliest. Combined with the contradictions of the stories about what exactly Newton deduced from this sudden inspiration - the direction of gravity, as per Stukeley? or the extent, as per Conduitt? or the ultimate cause, as per Voltaire? - there is deep suspicion among biographers of Newton about the accuracy of the tale.
Perhaps it served Newton to aggrandise himself with this story. Perhaps he was seeking to hide traces of disreputable alchemical thinking in his science. Or perhaps an old man simply had a romanticised memory of his youth. But whatever the truth, it seems clear that the story is, at best, an exaggeration.