In his new book, “In Pursuit of Civility”, British historian Keith Thomas tells the story of the most benign developments of the past 500 years: the spread of civilised manners. In the 16th and 17th centuries many people behaved like barbarians. They delighted in public hangings and torture. They stank to high heaven. Samuel Pepys defecated in a chimney. Josiah Pullen, vice-principal of Magdalen Hall, Oxford, urinated while showing a lady around his college, “still holding the lady fast by the hand”. It took centuries of painstaking effort – sermons, etiquette manuals and stern lectures – to convert them into civilised human beings.

Reading Thomas’s book on a train recently I was gripped by a terrible realisation: everything our forebears worked so hard to achieve is now reversing. A process that took centuries has been undone in just a few decades.

There is no better place to observe the collapse of manners than on mass transport. The most basic move in the civilising process was to make a distinction between the public and the private: persuading people to defecate in lavatories rather than chimneys and eat at regular times in designated places, not whenever or wherever the mood took them. Yet today city streets reek of urine and trains smell of fast food. I recently had the misfortune to sit next to a quivering man-mountain on a train who proceeded to slurp a Coke, demolish a Big Mac, munch fries and spill ketchup onto his beard while giggling at a film on his super-sized iPad. His only concession to the fact that he wasn’t in his own sitting room was to wear headphones.

Overnight flights are worse. I’ve never witnessed anybody urinating on the back of an airline seat, as apparently happened on a Frontier Airlines plane this May. But I’ve watched a man next to me floss his teeth and then carefully place the thread on the tray table, another do a vigorous push-up routine in the corridor, and a modern-day Henry VIII discard his chicken bones on the floor.

Enlightenment philosophers were convinced that the great engines of modernity – urbanisation, commerce and travel – would also spread civilisation. Commerce was supposed to polish people’s manners as well as fill their pockets. The closer association of people with each other would allow the masses to learn refinement. Today those very engines are turning against the civilising process. San Francisco is at the centre of the biggest creation of wealth on the planet, yet its streets are often littered with faeces, garbage and syringes.

The people who were supposed to act as guardians of high culture have collectively turned against it. Psychotherapists disparage self-restraint as a sign of unhealthy hang-ups. Academics are now so keen to denounce bourgeois civilisation as a tool of exploitation, patriarchy and/or misogyny that it can only be a matter of time before they start behaving like Josiah Pullen. One Cambridge economist, Victoria Bateman, turned up to a faculty meeting naked in protest at Britain’s vote to leave the EU. No wonder that today’s young, the most educated generation in history, are more likely to model their style on the urban underclass than on yesterday’s educated elite – hence the spread of tattoos, piercings and beards.

Civilisational decline is contagious: however hard you try to preserve your own manners you can’t resist the general trend. Make way for someone in a queue at Starbucks and you’ll have to wait ages as they order some ridiculously convoluted drink and then take the only free table. Keep the seat next to you clear on a train and you’ll find yourself sitting next to somebody who decides to treat it like their own sofa.

I don’t think I’ll ever give in to the fashion for beards and tattoos, let alone turning up to meetings naked. But I’ve noticed that I increasingly circumvent the normal rules of politeness in a desperate attempt to keep going. I carry a pair of headphones with me at all times to insulate myself from the noise of my neighbours. I sprint ahead if I see any possible competitors approaching the queue for coffee. And I’ve graduated from putting my bag on the seat next to me on a train to a more cunning technique: I leave a copy of Jack Rosewood’s “The Big Book of Serial Killers” on the chair and smile manically at anyone who comes anywhere near. So far the serial-killer strategy has worked remarkably well.