With the publication of the complete Principia in 1687, Newton reached the peak of his scientific career; he was ready to take a new direction in life. He no longer found contentment in his position in Cambridge; Trinity College had been in decline for some time, and Newton sought broader vistas. Thus he began taking on new roles: in 1687 he had his first taste of public life when the University sent him as part of a delegation to the new king, James II, to protest the king's attempt to allow a Benedictine monk to take a degree at Cambridge--a right long forbidden to Roman Catholics, who were an oppressed minority in Newton's England. The protest failed, for James was a fervent Catholic himself; however, the new king's sympathy for the forbidden religion led to his downfall in the Glorious Revolution of 1688. The English nobility invited William of Orange, a committed Dutch Protestant married to James' daughter Mary, to take the throne of England; the luckless James fled to France. Protestantism, embodied by the Anglican Church, once again secured its hold on power.

In this new protestant political climate the staunchly anti-Catholic Newton became an ever more public figure, becoming the representative to Parliament from Cambridge. In this position he did little to distinguish himself, though he won re-election in 1701. Meanwhile, he hoped for a higher appointment from the two monarchs, William and Mary, but in the early 1690s none was forthcoming. Some scholars suggest that the disappointment at not obtaining such a post might have contributed to what seems to have been a nervous breakdown in Newton; in 1692 he began a period of instability that would last until September 1693; even his closest friends believed him to have taken leave of his senses. Other possible factors in his affliction include mid-life crisis, a slow recession from the spotlight following the publication of Principia, and the collapse of his friendship with a young disciple with the unlikely name of Fatio de Duillier. Whatever the reason, Newton's condition seems to have struck with great force--he suffered from insomnia and depression, and his sensitivity to criticism gave way to a debilitating paranoia; it is referred to as his "Black Year." He lashed out at his friends, accusing them of conspiring against him; he wrote to John Locke, the great philosopher, claiming absurdly that Locke had tried to "embroil" him with "women and other means." News of Newton's collapse spread far and wide: Christian Huygens, the great astronomer, heard that "the illustrious geometer, Isaac Newton, had become insane." However, Newton recovered reasonably quickly, wrote letters of apology to his friends, and was back at work within a few months. Yet the period of breakdown seemed to have longer-lasting effects: while Newton emerged from the period with his faculties undiminished, he had seemingly lost interest in scientific problems; he now favored more arcane pursuits--notably the interpretation of prophecy and scripture, and the study of alchemy, the medieval pseudoscience that sought to transmute base elements into gold.

These interests have long puzzled those accustomed to viewing Newton as the dispassionate, rationalistic founder of modern science. The author of Principia left behind seemingly endless writings on subjects that his heirs consider beneath his genius--650,000 words on alchemy, and more than a million on religious topics. But in pursuing these concerns Newton was reflecting the times he lived in: in the turbulence of late 17th-century Britain, the Puritan legacy, with its emphasis on biblical literalism and the imminence of Christ's Second Coming, remained very strong. Even the most learned men anxiously anticipated the end of the world, and events like the plague and the great London fire of 1666 were considered harbingers of the apocalypse. People greeted the comet of 1680 was as another such sign, and many of the members of the Royal Society occupied themselves with attempts to date the Last Judgment. Robert Boyle, the great chemist, confidently anticipated cataclysm, predicting that the world would either descend into nothingness or transform in such a way as to "destroy the present frame of nature." Newton shared this view; indeed, even as he strove to understand the "present frame of nature" he also took the Bible literally and laboriously worked to link the prophecies in the books of Daniel and Revelation to historical events. He was not an orthodox Christian--his own logic led him to subscribe, in private, to a form of the Arian heresy, which denied the Trinity and held that Jesus Christ, while the Son of God, was not equal with the Father. (Arianism was also the system favored by John Milton, the century's greatest English poet.) But Newton's unorthodox beliefs did not prevent him from making endless speculations on scripture: he penned commentaries on Revelation, linking the antichrist to the Roman Catholic Pope, and encouraged a friend to write a book that made an attempt at proving, mathematically, the date of the Second Coming. (He himself declined to make any such pronouncement, however, saying he would "Let time be the interpreter.")

Meanwhile, Newton's interest in alchemy, despite endless experimentation, bore little or no fruit--he was perhaps the last great mind to pursue this quasi- magical discipline, and he did so with an odd credulity, eagerly embracing each new idea, only to reject it once his rigorous experiments had proven it false. Thus in later life, his enthusiasm for alchemy would wane even as his interest in scripture grew stronger; while his speculations on religion could not be disproved, he had met with too many disappointments in his quest for the secrets of transmutation. But his experiments reveal an important point--in Newton's time, modern science was still in its infancy, and the lure of magical thinking remained powerful. As C.S. Lewis has put it, "(The scientist's) endeavor is no doubt contrasted in our minds with that of the magicians: but contrasted only in the light of the event, only because we know that science succeeded and magic failed. That event was [in Newton's day] still uncertain." Thus in a way, it was not only the successes of Newton in physics and mathematics, but also his failures in alchemy, that solidified the triumph of science and the coming of modernity.