Everyone knows the rules. London invented trains that run underground as well as the protocol that makes the system bearable. A London Tube passenger enjoys the same prerogatives as an evil tyrant in a fairy tale. Do not stare. Do not approach. Do not distract him in any way. Respect the cocoon of solitude that protects him from the surrounding hubbub. But this simple covenant is not universally observed. Since I moved to London in 1973, I have twice been bothered by chatterboxes on the Tube. Twice, yes. It may sound like a minor annoyance to an outsider but residents of London will recognise the agony I have suffered.

The first intrusion occurred in 1991 as I sat on the District line at Earl’s Court. A fat drunkard elbowed my arm and asked me where the train was heading. I didn’t appreciate his physical approach and I smiled blithely at him. ‘I don’t know. No one knows where it’s going.’ This backfired instantly. ‘What’s that mean?’ he snarled in a coarse Ulster accent. ‘No one knows? What do you mean by that?’ An older passenger took a step forward and pacified my rowdy inquisitor with a word of warning. The drunkard got up and stumped off the train, muttering. Calm returned to the District line.

After this I enjoyed 22 years of undisturbed travel until 2013 when I found myself on the Central line opposite a tall Chinese tourist with a stout suitcase. The train doors slid open and I leaned out to enjoy the cooling gusts that race along the platform. The alarms beeped to indicate that the doors were about to close and I remained still, my head half in and half out of the carriage, enjoying the breeze until the very last moment. I sensed eyeballs on me. A human stare. The Chinese tourist was looking at me. ‘Mind your head,’ he said. I was outraged. I glared at him malevolently and tried to drill through his eyes and into his brain. He had broken the code. He had blasphemed against London. He had insulted the genius of this great city. Worse, he had forced me to acknowledge his fatuous warning in case he repeated it. ‘I’ve been on the Tube before,’ I said. The doors slid shut and the train moved off. I instantly felt a pang of remorse for my cutting words. Perhaps he was trying to be kind. Or maybe he was mentally ill. And yet he had chosen to shatter my solitude. And so he deserved the pitchfork treatment.

Last week it happened again. The Circle line, Saturday evening, not crowded. I took a seat beside a young man with a moustache, reading a paperback. I opened my paperback, noting to myself that two adjacent passengers reading paperbacks was unusual. I felt his eyes swivel towards me and I guessed my book’s cover had caught his attention. It had. ‘How are you finding it?’ he asked. I studied him. A fat stupid face, aged about 30, Irish accent, genteel Dublin, perhaps. My eyes burned into him but he failed to detect my pulses of hate. ‘You may be aware,’ he said, amplifying the terms of his overture, ‘that there’s been a fair amount of controversy about the book, and the film as well, and I wondered if you had any thoughts’.

A Tube passenger enjoys the same prerogatives as a tyrant in a fairytale. Do not stare. Do not approach

The book, The Salt Path, had caused a flurry of excitement when the author was found to have included details that varied from an earlier account of her life. I could have discussed this topic for ten minutes quite happily – but not under hostage conditions. I feel that a travel writer is free to use any resource at their disposal, recollection, omission, embellishment and so on. Factual accuracy belongs to affidavits or police reports. But my immediate problem was to vaporise the clingy Irish pest. ‘I know it’s fiction,’ I said tersely. He didn’t take the hint and he attempted for a third time to start a literary symposium. As he prattled away, I glanced at the covers of the paperback which I’d picked up from a garden wall a few days earlier. It was brand new. Aha. That explained it. My tormentor had assumed I’d selected the book at Waterstones or Foyles. But I gather my reading material in a more haphazard way than most. I help myself to spare paperbacks at random from an open-air library near my local park. His rabbiting question ended and I explained my position. ‘I don’t buy books,’ I said. ‘I pick them up from the street.’ This truthful statement made him chuckle nervously. ‘Oh right,’ he said, and the chuckle gave him a pretext to withdraw and to cease his attack on my seclusion. But God, I cursed his barbarous effrontery.

Am I overreacting? All he did was ask a question about a book that didn’t interest me much. And yet I found his crime unforgiveable. Freedom of thought is far more precious than freedom of speech. I happen to enjoy thinking – that is, emptying my brain and studying whatever floats through – but this fragile liberty is in constant danger. My only hope is to retreat to a soundproof oxygen-pod on a Norwegian mountainside. And even there, I suspect, some attention-thief would find me out and thump on my windows, demanding answers.