I first heard it from a man who did not know he was telling it.

It was in the reading room of the Bodleian, that great stone throat of a building that swallows sunlight and breathes it back as dust and footnotes. I had come to consult a parcel of nineteenth-century surveyor's maps — Ordnance Survey sheets, quarter-inch to the mile, showing Oxford as a grid of rational lines before the motorways came and unmade the country. The maps were harmless enough. They showed rivers as thin blue threads, railways as black stitches, hedgerows as dotted green seams. They showed the world as something you could hold in your palm and understand.

I was wrong about that, of course. You cannot hold a world in your palm. You can only hold the lines that describe it.

The man sat at the table across from me. He was the sort of person Oxford produces in abundance and then forgets to mention in its guides — middle-aged, tweed jacket with patched elbows that had never been mended, hair the colour of old newspaper. He was reading something with intense concentration, a slim volume with a cracked spine and no title on the cover. When he looked up and caught me glancing, he smiled the particular smile of someone who has been caught doing something mildly improper and is not sorry about it.

"You're looking at the maps," he said. It wasn't a question.

"I am. Surveyor's work. County boundaries, drainage patterns."

"Ah. Lines." He closed his book. "Lines to tell you where things are. Or where they used to be."

I should have let it go there. It was the kind of remark that dissolves if you pay it attention, like sugar in hot tea. But I am, by temperament and profession, a man who pulls at threads.

"What were you reading?" I asked.

He held up the slim volume. "A collection of jokes," he said. "Old ones. English jokes. The sort your grandfather told at Christmas and your father pretended not to hear."

Jokes. In the Bodleian. The incongruity of it struck me like a small physical blow. This was a temple of accumulated knowledge, a cathedral of citation, and here was a man reading jokes with the seriousness of a theologian poring over scripture.

"What kind of jokes?" I asked.

"The distant kind," he said, and returned to his reading as though he had not just placed a peculiar object on the table between us.

But I was listening. The word had landed somewhere behind my ribs and was echoing. Distant jokes. What does that mean, a joke that is distant? A joke told far from its audience? A joke that has grown old and faded at its edges? A joke that lives in another country and arrives here by mistake?

I wanted to ask him. But he had retreated behind his book the way a soldier retreats into a trench, and I had my maps to examine.

The maps showed a village called Standford on the Wye — Wye, not Way, though the river might as well have been called Way, because that is what it felt like, a long and patient going-toward-something that never quite arrived. The village sat on the border between Herefordshire and Monmouthshire, a border drawn in the nineteenth century by men who believed that a ridge of hills could hold a nation's disagreements. The river Wye ran through the village like a thought running through a mind — uncertain, circling back on itself, occasionally breaking into something that might be called music.

I was tracing the river with my finger when the man spoke again, without looking up from his pages.

"Do you know what the funniest thing about jokes is?" he asked.

I looked at him. "That most of them aren't funny?"

"That they require distance to work. A joke told too close becomes just a statement. You have to be a certain distance away from the truth for it to become laughable. That's all a joke is — truth, at the right distance."

He turned a page. The sound was like a bird taking off.

"I thought jokes were supposed to be about surprise," I said.

"Surprise is the mechanism. Distance is the condition. You can't have one without the other. You have to be surprised by something that you're also distant from. That's why we laugh at jokes about other countries, or other centuries, or people we'll never meet. We're safe in our distance. The joke is a protected space where truth can appear without demanding anything from us."

I looked down at my maps. The lines seemed less certain now, less like descriptions and more like suggestions — tentative proposals for how the world might be arranged. Rivers could go here or there. Roads could curve or straighten. Borders were not facts but opinions drawn in ink.

"What's your favourite joke?" I asked. It felt absurd, asking a stranger in a reading room about his favourite joke, but the question had taken root and was asking to be watered.

He thought about it for a long time. When he spoke, his voice had changed — softer, less certain, as though the joke lived somewhere deep and tender inside him and he was reluctant to expose it.

"There's an old one," he said. "About two friends walking through the countryside. One of them stops and says to the other, 'I've got a joke that would make you laugh more than any joke ever told. Want to hear it?' And the other says, 'Yes, I want to hear it.' And the first one says, 'No, you don't. You think you want to hear it. But you don't. Because once you've heard it, you can't unhear it. And the laughing — the laughing is just a way of covering up the fact that the joke has changed something in you that you can't change back.'"

The reading room had grown very quiet. Other readers had gone about their business — turning pages, whispering to librarians, making notes in margins — but at our table there was a silence that felt almost physical.

"That's not really a joke," I said.

"Isn't it?" He was smiling again, but this time there was sadness in it, the particular sadness that comes when someone has told you something true and knows you don't want to hear it. "It has all the elements. A setup. A twist. A punchline that lands somewhere you didn't expect. The only difference between that and the sort of jokes people tell at Christmas is that this one doesn't let you off the hook at the end. The laughing stops and you're left with the truth you were laughing at."

I closed my map. The village of Standford on the Wye disappeared beneath the leather binding, and with it went the comforting fiction that you could draw a line around a place and know where it ended.

"What's your name?" I asked.

"Thomas," he said.

"Thomas what?"

"That's all it is. Just Thomas. No surname. Never had one, as far as I can remember."

He stood up and gathered his things — the slim volume of jokes, a canvas bag that contained nothing I could see, a handkerchief folded into a precise square. He put the handkerchief away with the care of someone who stores something fragile for later use.

"Thank you," he said.

"For what?"

"For asking. Most people hear a sentence like 'distant joke' and move on to the next thing. You pulled at the thread."

He walked away through the stacks of tables and reading lamps, and I watched him go the way you watch a train disappear around a bend — knowing you'll never see it again, knowing that something about it will stay with you long after the sound has faded.

I sat there for a long time after he left. The maps were spread out between us, and I couldn't bring myself to pack them away. They showed a world that could be known — rivers, roads, borders, villages — and I had come to consult them as though knowledge were a matter of reading lines correctly. But the man with the jokes had understood something I was only beginning to grasp: that all maps are jokes, in the end. They're truths told from a distance, arrangements of ink that only work if you're willing to stand far enough away from them to pretend they're not made of nothing but suggestions and guesses.

The Wye ran through Standford and on toward the sea. On the map, it was a blue thread, patient and uncertain. In the world, it was cold and fast and full of things moving beneath the surface — stones, fish, the reflections of trees that didn't know they were being reflected, the occasional piece of rubbish floating downstream from someone else's carelessness.

I thought about the joke about the two friends and the joke that changes you. I thought about Thomas, who carried his collection of jokes like a man carries water in a cracked vessel — knowing it will leak, knowing it will diminish, but carrying it anyway because that's what carrying is, and nothing more.

And I thought about distance — the distance required for a joke to work, the distance required for a truth to become bearable, the distance between two people sitting at the same table in a reading room who will never see each other again but who have, for thirty minutes, shared the same quiet understanding that the world is not what the maps say it is.

The world is a joke told at a distance. And we are the audience, laughing at something we can barely see, barely understand, barely bear to look at directly. But we laugh, because laughing is the only response that lets us keep looking.

I packed away the maps. Standford on the Wye folded itself into leather and disappeared. I would never go there. I would never walk the streets that the surveyor's men had drawn with such careful, certain lines. But something about the village — the river, the border, the name that sounded like stand forward, like stand and face what you've been avoiding — stayed with me the way the man's joke stayed with me, like a truth carried at a distance that somehow feels more honest than if it were right in front of you, demanding you do something about it.

I stood up, left the reading room, and walked out into the Oxford afternoon. The sky was the colour of old paper, the same colour as the pages in Thomas's joke book. For a moment I thought he might be sitting at one of the tables outside the Bodleian, reading, waiting for someone to ask him about distant jokes.

But the table was empty. And the joke, like everything else, was just a little more distant than it had been before.