And it was then that he looked at me. At first he noticed my unclean clothes and blackened hands, and might have wondered what a rough-dressed man like myself was doing in his office.

‘I settled the payment downstairs,’ I said, clearing my throat. ‘I am now wondering if you recognise me.’

He looked up. His eyes met mine.

‘The last time I came to see you I did so under the name of Edward Cribbs. Do you remember that name? Do you remember? You advised me to go to the lunatic asylum.’

The rasping of his breath grew louder. He stood up out of his leather chair and came over to me. He stood ten inches from my nose. He rubbed those aged eyes.

A whisper. ‘No.’

‘You remember, don’t you? You do. I can see. Thirty-one years ago.’

He was out of breath, as if the realisation was a hill he had climbed. ‘No. No, no, no. It can’t be. It is an illusion. You might be Maskelyne or Cooke.’ (Maskelyne and Cooke were the illusionist double act of the day, who’d just been doing a host of London shows.)

‘I assure you it is I, sir.’

‘I must have taken leave of my senses.’

It was depressing that he found it so much easier to question his sanity than my reality.

‘No, sir, I assure you that you haven’t. The condition I told you about, my condition, the condition of holding back the tide of years, the condition that sounds like a blessing but which is also a curse – is real. I am real. My life is real. This is very real.’

‘You are not a ghost?’

‘No.’

‘You are not a spectre of my mind?’

‘No.’

His hand reached out to touch my face.

‘What was the day of your birth?’

‘I was born on the third of March in the year fifteen eighty-one.’

‘Fifteen eighty-one.’ He repeated it not as a question but as something so incredible it needed saying before it could be absorbed. ‘Fifteen eighty-one. Fifteen eighty-one. You were eighty-five years old when the Great Fire of London—’

‘I felt its heat. Its sparks singed my skin.’

He stared at me in a new way, as if he was a palaeontologist and I was a fresh dinosaur egg, ready to hatch. ‘Well, well, well. This changes everything. Everything.

‘Tell me, are you the only one? Have you ever known anyone else like you? With this . . . condition?’

‘Yes!’ I said. ‘There was a man I met once, during Captain Cook’s second voyage. A man from the Pacific Islands. His name was Omai. He became the rarest of things – a friend to me. And also . . . my daughter Marion. I have not seen her since she was a girl. Her mother told me that she had inherited my condition. That she stopped ageing normally around eleven years of age.’

Dr Hutchinson smiled. ‘This is a gigantic thing to comprehend.’

And I smiled too, and felt the soul-anchoring joy of being understood.

And this joy stayed inside me right up until Dr Hutchinson’s body was found floating in the Thames thirteen days later.

London, now

I still have a headache.

Sometimes it is almost not there, while at other times that is all there is, and the pain always coincides with memories. It is less a headache and more a memory ache. A life ache.

No matter what I do, it never goes completely. I have tried everything. I’ve taken ibuprofen, drunk litres of water, had lavender-scented baths, lain in the dark, rubbed my temples in slow circles, slow-breathed, listened to lute music and the sound of waves on a beach, meditated, did a stress-relief yoga video course where I repeated the mantra ‘I am safe, it’s okay to let go’ about a hundred times until I felt terrified of my own voice, watched brain-dead TV, stopped drinking caffeine, turned the brightness down on my laptop, but still the headache stays, as stubborn as a shadow.

The one thing I haven’t properly tried is sleep. I have a trouble with sleep that has been growing over the decades.

Last night I couldn’t sleep so I watched a documentary about turtles. They aren’t the longest-living species but they are one of them, and some turtles ‘live to over one hundred and eighty’. I put that in inverted commas because mayfly estimates such as these always turn out to be underestimates. Just look at how wrong they were about sharks. Or, well, humans. My bet is that there is at least one turtle out there approaching her five hundredth birthday.

Anyway, the thing that was depressing me was that humans weren’t turtles. Turtles have been around for two hundred and twenty million years. Since the Triassic period. And they haven’t really changed that much. Humans, in contrast, have been around only a short while.

And you don’t have to be a genius to switch on the news and conclude: we probably don’t have long. The other human sub-species – such as the Neanderthals, the Denisovans in Asia, the casually named ‘hobbits’ of Indonesia – had proven crap at the long game and so, most likely, would we.

It is all right for the mayflies. It is all right if you know you only have another thirty or forty years. You can afford to think small. You can find it easy to imagine that you are a fixed thing, inside a fixed nation, with a fixed flag, and a fixed outlook. You can imagine that these things mean something.

The longer you live, the more you realise that nothing is fixed. Everyone will become a refugee if they live long enough. Everyone would realise their nationality means little in the long run. Everyone would see their worldviews challenged and disproved. Everyone would realise that the thing that defines a human being is being a human.