The mathematician was alone when he died in hospital; after several weeks in their company, he had spurned his children again, only accepting care from intermediaries. The presence of his family seemed to stir up unbearable feelings. In his writings, he evaluates the people in his life for how much they are under the sway of Satan. But, as Alexandre points out, this was also a projection of his own seething unconscious: “He didn’t like what he saw in the mirror we held out to him.” An elderly Alexander Grothendieck, with a long grey beard and glasses, in bed, wearing a wooly hat, with a painting of another man on the wall behind him View image in fullscreen Grothendieck at home in Lasserre, France, shortly before his death in 2014. Behind him is a painting of his father. Photograph: Matthieu Grothendieck They only discovered his whereabouts in Lasserre by accident: one day in the late 90s Alexandre signed up for car insurance, and the company said they already had an address for an Alexander Grothendieck on file. Deciding to make contact, Alexandre spotted his father across the marketplace in the town of St-Girons, south of Lasserre. “Suddenly, he sees me,” says Alexandre. “He’s got a big smile, he’s super-happy. So I said to him: ‘Let me take your basket.’ And all of a sudden, he has a thought that he shouldn’t have anything to do with me, and his smile turns the other way. It lasted a minute and a half. A total cold shower.” He didn’t see his father again until the year he died. At least until the early 00s, Grothendieck worked at a ferocious pace, often writing up the day’s “meditation” at the kitchen table in the dead of night. “He became totally isolated. He was no longer in contact with nature. He had cut ties with everyone,” says Johanna. He vacillated about the date of the Day of Liberation, when evil on Earth would cease. Recalculating it as late August or early September 1996 instead of the original October date, he was crestfallen at the lack of celestial trumpets. Mathematicians Leila Schneps and Pierre Lochak, who had tracked him down a year earlier, visited him the day afterwards. “We delicately said: ‘Perhaps it’s started and people’s hearts are opening.’ But obviously he believed what we believed, which was that nothing had happened,” Schneps says. Experiencing an “uncontrollable antipathy” to his work, that he attributed to malign forces but sounds a lot like depression, he wrote in early 1997: “The most abominable thing in the fate of victims is that Satan is master of their thoughts and feelings.” He contemplated suicide for several days, but resolved to continue living as a self-declared victim. The house was weighing on him. In 2000, he offered it to his bookbinder, Michel Camilleri, for free, deeming him the perfect candidate because he was “good with materials”. The sole condition was that Camilleri look after his plant friends. When Camilleri refused, he was outraged – seeing the hand of Satan once more. A year later, the building was nearly destroyed when his unswept stove chimney caught fire. Some witnesses say Grothendieck tried to prevent the firefighters from accessing his property (Matthieu doesn’t believe this). Some open shutters on a house totally obscured by creepers View image in fullscreen Grothendieck’s house in Lasserre. Photograph: Ulrich Lebeuf/MYOP/The Guardian The curate at Lasserre church, David Naït Saadi, wrote Grothendieck a letter in around 2005, attempting to bring the hermit into the community. But Grothendieck fired back a missive full of biblical references, saying Saadi had a “viper’s tongue” and that he should nail his reply to the church noticeboard. By the mid-00s, his writing was petering out. The endpoint of his late meditations, according to Matthieu, is a chronicle in which his father painstakingly records everything he is doing, as if the minutiae of his own life are imbued with immanence. Matthieu finds these writings so painful to read that he kept them back from the national library donation. Grothendieck was lost in the rooms and corridors of his own mind.