In 1687, over a year later Halley persuaded Isaac to write it, he published Philosophiae naturalis principia mathematica or Principia, a full treatment of Newton's new physics and its application to astronomy.
The Principia is recognized by many to be the greatest scientific book ever written. In it Newton analyzed the motion of bodies in resisting and non-resisting media under the action of centripetal forces. Results were applied to orbiting bodies, projectiles, pendulums, and free-fall near the Earth. He further demonstrated that the planets were attracted toward the Sun by a force varying as the inverse square of the distance and generalized that all heavenly bodies mutually attract one another. Further generalization of this led to the law of universal gravitation which Newton described as:
... all matter attracts all other matter with a force proportional
to the product of their masses and inversely proportional to
the square of the distance between them.
Newton became an international leader in scientific research because of his explanations in a wide range of unrelated phenomena; the eccentric orbits of comets, the precession of the Earth's axis, the tides and their variations, and motion of the Moon as perturbed by the gravity of the Sun. The continental scientists however continued to believe in Descartes' vortex theory, but still it did not stop the universal admiration of Newton's technical expertise.
On January 15th 1689, the University of Cambridge elected Isaac Newton as one of their two members to the Convention of Parliament. Newton was at the highest point in his standings, being seen as a leader of the University and one of the greatest mathematicians in the world. However it is thought that this election to Parliament may have allowed him to see that there was a life in London that might appeal to him more than the academic world in Cambridge.
Newton suffered a second nervous breakdown in 1693, and retired from research. Many observations have been made as to the cause of this breakdown ranging from chemical poisoning as a result of experiments in alchemy, frustration with his researchers and even problems resulting from his religious beliefs. Newton himself simplified it, saying that it was a result of lack of sleep. But this was more likely a symptom rather than the cause. It seems that the true core of his illnesses was depression, a mental illness he must have suffered through most of his life.
In light of Newton's tendencies to rage at any form of criticism, it isn't surprising that he flew into a typical rage at Leibniz. Newton used his position at the Royal Society by appointing a supposedly impartial committee to decide who the true inventor of calculus was. Newton even wrote the official report of the committee, under another name of course, which was published by the Royal Society. Then after he wrote a review of these finding, again under an assumed name, which was published in Philosophical Transactions of the Royal Society. In reference to his rage, his assistant Whiston later wrote; "Newton was of the most fearful, cautious and suspicious temper that I ever knew."