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Edmund Halley paid for the Principia himself when the Royal Society said it could not afford it. The society had spent its budget on a book about the history of fish. I am grateful to Halley. Without him the Principia might have remained a pile of manuscripts in my rooms at Trinity.

I published in 1687. Three books: the laws of motion, the mathematics of orbits, and the system of the world. The third book demonstrates that the same force that causes an apple to fall causes the moon to orbit the earth and the planets to orbit the sun. There is one law. It applies everywhere.

Hooke wrote to claim priority. He said I had taken his ideas on gravitation. I will say only this: Hooke had the notion that gravity might follow an inverse-square law. I proved it, derived the orbits from it, and calculated the mass of Jupiter. Having the notion and proving it are different things. I removed his name from the acknowledgements.