By the end of his life, Newton was one of the most famous men in England, his pre-eminence in matters scientific unchallenged. He had also become a wealthy man; he invested his substantial income wisely, and had enough to make sizable gifts to charity and leave a small fortune behind in his will. Whether he was happy is another question. He had never made friends easily, and in his later years his peculiar combination of pride, insecurity, and distraction seems to have interfered with his relationships. He never married, and lived as the "monk of science," having channeled all his sexual energy into his work. His only close relationships with women were familial: with his niece, with whom he lived for some years, and much earlier, with his mother, who had died in 1679. Around 1700 he had briefly courted a wealthy widow, but nothing came of it.

In old age Newton's health began to deteriorate: whe was eighty he began to suffer from incontinence, due to a weakness in the bladder, and his movement and diet became restricted. He ate mainly vegetables and broth, and was plagued by a stone in the bladder. In 1725 he fell ill with gout, and endured hemorrhoids the following year. Meanwhile, the pain from his bladder stones grew worse, and on March 19, 1727, he blacked out, never to regain consciousness. He died on March 20, at the age of eighty-five, and was buried in Westminster Abbey; his funeral attended by all of England's eminent figures, and his coffin carried by noblemen. It was, a contemporary noted, a funeral fit for a king.

His fame only grew with his death. Decades later, the philosopher David Hume would write that Newton was "the greatest and rarest genius that ever rose for the adornment and instruction of the species." Alexander Pope, the great English poet, composed an epitaph: "Nature and Nature's laws lay hid in night; / God said, Let Newton be! and all was light." This was an exaggeration, of course; Newton's achievement was not a burst of light against the darkness, but rather one explosion among many in the progress of the Scientific Revolution. But his was the greatest explosion, by far, and Newton's impact on the world of Western thought can be compared to the impact of figures like Plato, Aristotle, Galileo, and even Jesus. Not every idea he pursued led to a triumph; his mathematical systems proved somewhat less successful than those of Leibniz, and his endless writings on alchemy and theology languished, and are now read only by biographers seeking to better understand this complex, contradictory man. But Newton's triumphs, and the universal principles they uncovered, found no parallels in the science of his time. As the French thinker Laplace was to remark, a trifle regretfully, there was only one universe, so only one man could discover its "fundamental law." That law was gravity, and that man, for hundreds of years, was Isaac Newton.

In the end, of course, Laplace was proven wrong. In the 20th century, Albert Einstein would overturn the Newtonian understanding of the universe, showing that the things that Newton had considered absolute--space, distance, time, motion--were in fact relative. Einstein would show that space and time were one fabric, known as "space-time," that the universe was a wider and more fantastical place than Newton had thought possible, one in which formulae and unified laws could no longer hold true. And yet, perhaps these subsequently- discovered wonders would not have surprised the great scientist. As an old man, when asked for an assessment of his achievements, Newton replied: "I do not know what I may appear to the world; but to myself I seem to have been only like a boy playing on the seashore, and diverting myself now and then in finding a smoother pebble or prettier shell than ordinary, while the great ocean of truth lay all undiscovered before me."