Tim Parks, the novelist and essayist, tells us that, for a long time, he suffered from pelvic pain. He had to get out of bed several times a night to go to the loo. He thought he might have a problem with his prostate or his bladder. This is how he describes his pain: "a general smouldering tension throughout the abdomen, a sharp jab in the perineum, an electric shock darting down the inside of the thighs, an ache in the small of the back, a shivery twinge in the penis itself".

He talks to his friend Carlo, a urologist. Carlo suggests an operation. He also suggests pills. But the pills make Parks constipated. He stops taking them. Result: "Normal bowel movements. Normal pains. It seemed like progress." He finds himself lying in bed, "rigid and angry". He's angry with the doctor, and with life in general. "My body seemed alien and malignant. We couldn't get comfortable together."

At this point, one imagines what the rest of the book will look like – Parks submits to the doctor, has the operation, and then several more operations. He looks back over his life as he feels it slipping away. As mortality looms, his memories take on the weight of poignance – and so on. But this is not that type of book. Parks has tests. The tests are vague and inconclusive. The pain does not go away. So he decides to think about the pain. Maybe it's all in his head.

He tells us about his hospital appointments in detail; there's a scene in which he tries, and tries, and tries, to wee into a test tube. He has various examinations. At this point, the reader is seriously hoping, for the sake of Parks but also for the sake of the book, that the problem is in his head, and not in his flesh. Parks goes to India and while he's there decides to see an Indian doctor. "There is a tussle in your mind," says the doctor. The pain is "blocked vata".

What happens next is a lovely, well-told story, an investigation into the relationship between mind and body, and a joy to read for anyone with hypochondriac tendencies. Parks, an inveterate reader of literary books, wonders if Thomas Hardy's pelvic pain was psychosomatic. Yes, possibly it was.

In 1880, Hardy wasn't getting on with his wife. They flitted around, moving from Dorset to London and back again. And then: the pelvic pain. Hardy's wife became his carer. They settled in Dorset. The pain sorted his life out.

And what about Mussolini? He suffered bouts of stomach pain, often after he'd comitted acts of political brutality. Doctors suspected an ulcer. But when he died, an autopsy revealed... nothing. His stomach was fine.

Parks ponders these matters, but still his terrible pains continue. "I was becoming withdrawn," he says. "I was losing interest in intimacy." Even the characters in his novels were displaying the same symptoms – "manic and withdrawn".

And then he discovers a book on the internet, called A Headache in the Pelvis. Written by two Californian doctors, it suggests that Parks might be suffering from nothing more than muscle tension. Can this be true? Can decades of agony and malaise be caused by sitting on your bottom and clenching your muscles? Yes, apparently.

About now, Parks has his spiritual breakthrough. He realises that, as a writer, he hardly ever lives in the moment – up to now, he's spent the vast majority of his time thinking about how to translate his experiences into words. He's been living in the past, and in the future, but never quite in the present. And this has caused his entire body to become rigid with tension. "I yanked on my socks as if determined to thrust my toes right through them. I tied my shoes as if intent on snapping the laces." He's tense when he shaves, when he eats, when he grips the steering wheel.

So he starts to meditate. He realises his problem; it's almost impossible to make his mind relax, because it's full of words. But he tries and tries. He meditates and exercises. He spends time at a retreat. Gradually, he clears his mind. The pain in his pelvis disappears. It wasn't cancer. It wasn't a bladder infection. But it wasn't nothing. It was the anxiety of the sedentary westerner who sits on his bottom for hours on end, racking his brains and tapping a keyboard.

A simple story, then, but very well told. Parks takes many detours through the worlds of literature and art. He tells us about Coleridge and his neuroses. He meets a strange old American guru called John Coleman. Parks is a conscientious and expert companion whom it is hard not to like. And, if you spend your time sitting on your bottom, rigid with tension, and you've been feeling a lot of aches and pains lately, you'll love his message. It is: relax. Sit still. Stop worrying.