In the summer of 1665, plague was killing thousands a week in London. Cambridge, downstream of the disease both physically and demographically, shut its colleges and sent its scholars home. Newton was a twenty-two-year-old undergraduate of unremarkable academic record. He went back to the family farm at Woolsthorpe, a typical 17th-century yeoman house of pale Lincolnshire limestone, with mullioned windows and a kitchen garden, surrounded by sheep pasture. His mother ran the place. He was supposed to be helping. Instead, in his bedroom on the upper floor, he set up a prism in the shutter so that a single beam of light could enter the dim room. He watched the colours spread across the wall. He watched them and watched them and watched them, and worked out that white light was a mixture of colours, not a pure thing as everyone had assumed, but a sum. He was twenty-three.

In the eighteen months that followed, Newton invented or set the foundations for what he would later call calculus - the mathematics of how things change, of curves, of motion, of any quantity that flows. He worked out the inverse-square law of gravity. He sketched the experiments on light and colour that would later become the Opticks. He did this alone, in a farmhouse, with no peers, no library beyond what he had brought from Cambridge, and no one to tell him these problems were too hard. His friend William Stukeley later wrote down what Newton had told him about the apple, sitting under the tree in the Woolsthorpe garden in the cool of an afternoon, watching one fall. Why does it fall straight down? Why not sideways? Why not up? Newton's mind ran the question out to the moon, which also falls towards the Earth but never arrives because it keeps missing - and the modern physical picture of the universe came into focus.

Woolsthorpe sits on the Lower Lincolnshire Limestone, the same pale Jurassic stone that built the manor house and that, a few miles north at Ancaster, was quarried and shipped as far as London. Beneath the limestone lies the Northampton Sand, cemented with iron, and that geology - so quiet under your feet today - shaped the hamlet's modern history. In the 20th century the fields around the village were strip-mined for ore. A branch railway, the High Dyke line, was built from 1916 and opened in 1919 to haul the ironstone out. The mines closed in 1973 when imported ore from elsewhere undercut Lincolnshire's product, and the railway closed the same year. The land has healed; the manor has not changed. Newton's mother's farmhouse stands quietly amid sheep and apple trees, the National Trust running it now, the bedroom where the prism stood preserved and the orchard where the apple fell maintained against the slow violence of time.

What is striking about Woolsthorpe Manor is how ordinary it is. This is not a palace. It is not a college, not a court, not the home of nobility. It is a working farmhouse of yellow stone with chickens and small rooms and a stone-flagged kitchen, the kind of place where, on a Lincolnshire afternoon in 1666, a young man with a notebook and a beam of light through a shutter changed the course of human understanding. The hamlet has no shop and barely a pub. The A1 thunders by a mile away, the same road that Roman legionaries marched along when they were building Ermine Street, the same road Dick Turpin galloped, the same road millions of cars now use to go to Scotland and back without stopping. Most of them never know that the man who explained why their cars stay on the road, why the stars stay in the sky, and why an apple falls down rather than up, sat in a small upstairs bedroom a mile to the east and figured it all out one summer.