Wood, he believed, was conscious. He told Michel Camilleri, a local bookbinder who helped compile his writings, that his kitchen table “knows more about you, your past, your present and your future than you will ever know”. But these wild preoccupations took him to dark places: he told one visitor that there were entities inside his house that might harm him. A bald man with glasses standing outside talking to another man, both smartly dressed View image in fullscreen Grothendieck, left, with fellow mathematician Laurent Schwartz. Photograph: IHES Grothendieck’s genius defied his attempts at erasing his own renown. He lurks in the background of one of Cormac McCarthy’s final novels, Stella Maris, as an eminence grise who leads on its psychically disturbed mathematician protagonist. The long-awaited publication in 2022 of Grothendieck’s exhaustive memoir, Harvests and Sowings, renewed interest in his work. And there is growing academic and corporate attention to how Grothendieckian concepts could be practically applied for technological ends. Chinese telecoms giant Huawei believes his esoteric concept of the topos could be key to building the next generation of AI, and has hired Fields medal-winner Laurent Lafforgue to explore this subject. But Grothendieck’s motivations were not worldly ones, as his former colleague Pierre Cartier understood. “Even in his mathematical milieu, he wasn’t quite a member of the family,” writes Cartier. “He pursued a kind of monologue, or rather a dialogue with mathematics and God, which to him were one and the same.” Beyond his mathematics was the unknown. Were his final writings, an avalanche of 70,000 pages in an often near-illegible hand, the aimless scribblings of a madman? Or had the anchorite of Lasserre made one last thrust into the secret architecture of the universe? And what would this outsider – who had spurned the scientific establishment and modern society – make of the idea of tech titans sizing up his intellectual property for exploitation? In a famous passage from Harvests and Sowings, Grothendieck writes that most mathematicians work within a preconceived framework: “They are like the inheritors of a large and beautiful house all ready-built, with its living rooms and kitchens and workshops, and its kitchen utensils and tools for all and sundry, with which there is indeed everything to cook and tinker.” But he is part of a rarer breed: the builders, “whose instinctive vocation and joy is to construct new houses”. Now his son, Matthieu Grothendieck, is working out what to do with his father’s home. Lasserre lies on the top of a hill 22 miles (35km) north of the Spanish border, in the remote Ariège département, a haven for marginals, drifters and utopians. I first walk up there one piercingly cold January morning in 2023, mists cloaking forests of oak and beech, red kites surveying the fields in between. Grothendieck’s home – one of Laserre’s few three-storey houses, build by affluent dressmakers – is at the village’s southern extremity. Hanging above the road beyond are the snow-covered Pyrenees: a promise of a higher reality. A tall man wearing a dark red T-shirt and beige shorts, standing on grass outside an old cream French house with shutters and with plants growing on it View image in fullscreen Matthieu Grothendieck outside his father’s house in Lasserre in the French Pyrenees. Photograph: Ulrich Lebeuf/MYOP/The Guardian Matthieu answers the door wearing a dressing gown, with the sheepish air of a man emerging from hibernation. The 57-year-old has deeply creased features and a strong prow of a nose. Inheriting the house where his father experienced such mental ordeals weighs on him. “This place has a history that’s bigger than me,” he says, his voice softened by smoking. “And as I haven’t got the means to knock it into shape, I feel bad about that. I feel as if I’m still living in my father’s house.” A former ceramicist, he is now a part-time musician. In the kitchen, a long, framed scroll of Chinese script stands on a sideboard, next to one photograph of a Buddha sculpture and two of his mother, Mireille Dufour, whom Grothendieck left in 1972. (Matthieu is her youngest child; he has a sister, Johanna, and brother, Alexandre. Grothendieck also had two other sons, Serge and John, with two other women.) Above Matthieu’s bed is a garish portrait of his paternal grandfather, Alexander Schapiro, a Ukrainian Jewish anarchist who lost an arm escaping a tsarist prison, and later fought in the Spanish civil war. Even with all his wisdom and the depth of his insight, there was always a sense of excessiveness about my father. He always had to put himself in danger Schapiro and his partner, the German writer Johanna Grothendieck, left the five-year-old Grothendieck in foster care in Hamburg when they fled Nazi Germany in 1933 to fight for the socialist cause in Europe. He was reunited with his mother in 1939, and lived the remainder of the war in a French int