Hodder £16.99 pp242

Daniel Tammet is a man with an exceptional mind: he can perform mental arithmetic faster than a pocket calculator, and learn a new language fluently, even one as tortuously complex as Icelandic, in a matter of days. Even more admirable, though, are his modesty and guilelessness, so evident in this memoir of outstanding lucidity and charm. One also admires the way in which, against all the commandments of our age, he has evaded becoming a celebrity. He has admittedly “performed” in Las Vegas, a little uncomfortably, and appeared in a television documentary last year punningly entitled Brainman, since the lazy shorthand for describing him is that he is “the real-life Rainman”. He tolerates the nickname with a certain wariness. But having had a taste of the limelight, he soon wanted to be back at home in Herne Bay with his partner Neil, growing his vegetables in the garden: “onions, peas, potatoes, tomatoes, cabbages, lettuces and herbs”, as he tells us with quiet pride. Not only an exceptional mind then, but an exceptionally sensible one as well.

Having Asperger’s, he is incapable of any elaborate games of image projection, deceit or sarcasm. It’s strangely moving and humbling to have someone telling it to you so straight. When he was younger a rather nosey woman once asked him about his personal life. “I did not have a personal life and so did not know what to say.” And recalling his childhood, “I would spend hours at night awake in bed looking up at the ceiling and imagining what it might be like to be friends with somebody.”

His mathematical abilities are awesome, incomprehensible. He doesn’t merely do sums extremely fast in his head, he sees them as richly coloured patterns and images (words are seen as colours, too, hence the title). Asked what 37 to the power of 5 is, he sees “a large circle composed of smaller circles running clockwise from the top round”, which somehow gives him the answer instantly, without calculation: 69,343,957. He is also brilliant at chess, winning his first two games, against his much more experienced father, after a brief explanation of the rules. A downside of his condition, however, is that he lacks the intense concentration needed to make a grand master.

As an isolated and obsessive little boy, he spent his time covering the inside of the garden shed with ancient Phoenician lettering, and was fascinated by the fact that all three of his childhood home addresses were prime numbers: 5, 43 and 181. He sees any prime number as a beautiful round pebble. At the same time he has virtually no sense of direction, gets lost easily and struggles to remember left from right. He becomes anxious if his routines are upset, the rigid order of his day beginning with exactly 45g of porridge for breakfast. More seriously, he knows that he has to work extra hard on “social interaction, communication and imagination”. He reads voraciously, but fiction is a closed book. He is baffled by idioms and contradictory language, such as people saying, “He’s not tall, he’s a giant”, and “Don’t you want some ice-cream?” But you do sense, hearteningly, that there’s no unbridgeable divide between Tammet and the rest of us: it’s a spectrum, after all, not a chasm. He says he gets agitated if he can’t drink his cups of tea at the same time daily, but so do an awful lot of British people. I get pretty touchy myself if I can’t get hold of a decent cuppa around 4pm.

Finding he was gay was one step towards understanding himself and finding greater contentment, and the correct diagnosis for Asperger’s when he was 25 helped, too. Since then he has started a highly successful internet-based course for language learners, and, in a final, fascinating revelation, become a committed Christian. For someone with such a logical and literal mind, how can this be? He says that he finds the concept of the Trinity mathematically appealing, but finally defines his faith as being about love. People like Tammet used to be called “idiot savants”, but the idiot bit has quite rightly been dropped. There is nothing idiotic about this man.

Daniel Tammet, A Life in the Day, ST magazine

Times Recommends

• Why do Pynchon, Ballard and Wallace provoke such online loyalty?
• The Sunday Times 100 best holiday reads
• What will we be reading this summer: the booksellers predict