Most important, this new quantitative approach to nature offered the gift of prophecy: calculate now, and you can find where the moon or Jupiter or whatever will be, days, years, centuries from now. Though Newton called what he did in that Lincolnshire farmhouse “natural philosophy,” this was science in its (early) modern form.

THERE WAS ONE more move Newton made to complete this new approach to nature. A system of equations could yield a portrait of some aspect of the wide world, but any such model still needed reliable knowledge extracted from the natural world to make the connection between math and experience. Observation, measurement, and especially a commitment to rigor were keys to Newton’s new approach to natural philosophy: it was essential to test the world precisely and reliably enough so that any mathematical analysis of what was going on could yield a real insight into reality. One of the reasons Newton is remembered as perhaps the greatest genius in history is that from the beginning he did just that, plunging deeply not just into mathematics but into measurement of the world—at times to his own peril.

Newton’s drawing of his bodkin experiment

For example, in the early 1660s he wanted to know how the shape of the human eye might affect the perception of color. To find out, he turned to the nearest experimental subject, himself, and stuck a bodkin—a blunt needle—into the bottom of his eye socket and levered up. He meticulously recorded his results, including defining the curve he induced in his eyeball (“ye curvature a b c d e f”) and noting that the colored circles grew brighter “when I continued to rub my eye with ye point of ye bodkin.” His sketch of what he did to himself is at once meticulous and stomach roiling, a measure of both the urgency of his hunger for data and his utter recklessness.

Such mad hunger would seize Newton over and over again—he would later pursue his alchemical experiments so relentlessly that he drove himself to the point of physical exhaustion and, at least once, in 1693, tipped all the way over the edge into true mental collapse, months of silence and paranoid misery. But such excess shouldn’t obscure his deeper and lifelong commitment to a cooler empiricism: the need to base any scientific speculation on the solid ground of rigorous and systematic observation.

That’s what occupied him in the last months of his plague exile: optical investigations that demanded hands-on effort in a series of organized, rigorous experiments with prisms and other apparatuses to tease apart the properties of color and light. The work ultimately produced what he called the experimentum crucis.That was his crucial demonstration—that sunlight, so-called white light, is actually a blend of distinct individual colors, the rainbow spectrum from red to blue.

The creation of new knowledge of the world required theories, ultimately mathematical accounts of the relationships between phenomena and events. It also needed a careful, logically coherent approach to the collection of data, observations and experiments that could probe the material world. As his miraculous year unfolded, Newton found himself tracing this arc: through each particular advance, from pure mathematics to what he was able to deduce as he twisted a triangular piece of glass, he uncovered not just previously unknown facts but a whole new way of organizing that knowledge into what he would later call, accurately, a “System of the World.”

BEYOND WOOLSTHORPE AND its scenes of revolutionary intellectual transformation—all completely unknown outside the farmhouse walls—the rest of Newton’s world, haltingly, began to reassemble itself. By mid-March 1666 the city of Cambridge marked six weeks without a single plague death. The university reopened, and Newton returned to his rooms on or around March 20. Then, on Wednesday, June 6, Jane Ellingworth, a seamstress living on Penny Farthing Lane, felt poorly. Her father brought her a cup of ale and urged her to bed. She died the next day. The infection spread, with deaths reaching double digits in the city within two weeks. The colleges closed again, and Newton retreated to the countryside once more. The plague burned itself out over the summer, and by the end of the year Newton felt it safe enough to travel back to the university. He would remain there for the next thirty years.

By the time Newton resumed his almost-cloistered life in Trinity College, the onetime center of the epidemic had been utterly transformed. London’s parish records reported a total of 68,596 plague deaths for 1665. The true figure was certainly higher.

Feel and make