Based on Galileo's experiments, Newton develops the theory of gravity in his first book Philosophiæ Naturalis Principia Mathematica ("Principia") of 1686. Immediately after, Robert Hooke accused Newton of plagiarism, claiming that he unduly assumed his "notion" of "the rule of the decrease of Gravity, being reciprocally as the squares of the distances from the Center". But, according to Edmond Halley, Hooke agreed that "the demonstration of the curves generated by it" belongs entirely to Newton.