Petra was brought into my office. There is something about it being the last day of your life which modulates every emotion: Petra looked scared, resigned, even – against the odds – slightly hopeful – but most of all she just looked bone-achingly weary. I stopped the nervous adjustment of my robes, calmed myself and looked her in the eye with what I hoped was a friendly expression.

‘You didn’t ask to see me, Petra, but … it’s a facility our hosts here grant us. First of all tell me, are you a Christian?’

Petra shook her head. Young people seldom think deeply about religion in my experience, and all this had obviously happened far too suddenly for Petra to get around to revisiting her core beliefs. Well, that was partially why I was here.

‘So when they take you out at dawn tomorrow and shoot you through the heart, that’s it? A kick in the chest, a brief moment of pain followed by gathering blackness, and then you never get to wake again. Is that right?’

Petra looked at her feet. People often turn in on themselves as the moment gets near and I guess the last thing she wanted today was another irksome, proselytizing priest.

‘Patience Petra, humor me. Just suppose that tomorrow morning, rather than facing a line of anonymous soldiers with their raised rifles, you were going to be pushed through some kind of matter transport device, you know, like in science-fiction? The device would remove your memories so you’d end up in some other place and time with someone else’s memories. Suppose they gave you that option, would you take it over the execution squad?’

Petra looked at me in contempt, the momentary flicker of hope on her face quickly extinguished. ‘There is no such device, you’re wasting my time, priest. But to answer your question, what could be the difference between dying and becoming someone else in the way you describe?’

You are not your memories, child. There have been cases of people who’ve developed amnesia and forgotten everything about their past. They did not wish they were dead; they carried on with their lives. What you truly are is your character, your personality and your evident intelligence.’

‘Yes,’ she said bitterly, ‘all the things they’re going to erase tomorrow morning at dawn.’

‘How many ways are there of being human, Petra? It’s far from infinite. Whether we’re talking about your IQ or the different facets of your personality, they’re not quite the same day-by-day or even hour-by-hour. They vary.

‘Have you ever met someone and thought that they’re exactly like someone else you already know? I have – it’s rather spooky. I once encountered a woman exactly like an office colleague I had worked with some years previously. In fact I had a brief fantasy that they should both meet and each recognize their own self in the other. And we’ve all heard about the man who married the same woman two or three times.’

Petra cut through this babble, finally looking me in the eye.’ You’re saying that there are people who are just like me out there in the world?’ She gave me a wry smile which in happier times would have been considered flirtatious, ‘I thought I was unique. But I can’t see that it matters, I’m me and they’re them. We’re not the same person.’

‘In a world of more than six billion people,’ I replied, ‘there are about a hundred people who would test indistinguishably from you on personality and smartness. When they get up in the morning, they feel like you do, they think like you do and they react like you do. If one of your friends met any of them, he or she would interact with that person just as they do with you.’

‘Or they would with the 10% of them of the same gender and in the same age-group as yourself,’ I added with a smile.

‘But I’m not them,’ Petra reiterated stubbornly.

‘I wonder how you know that,’ I mused. ‘When you wake up in the morning, your brain reconstructs your sense of yourself and loads it with your memories. Your psychological clones out there are doing just the same. In some sense they’re all you, just variants with different memories. One of you will get shot tomorrow morning – there’s nothing I can do about that, I’m afraid – but the other hundred of “you”s will just carry on with their lives. Some will be coming to their natural ends, others are just being born. In this sense, as long as the human race survives, the essential you is immortal.’

Petra looked at me skeptically, ‘I can’t blame you for trying but this is not going to help when I’m tied, blindfolded, to a stake tomorrow morning. It’s my body they’re shooting.’

‘Yes it is and your body will signify its fear to you. But as you wait, try this. Reach out to your other selves and feel your community in the world. Your body may be vanishing from that collective, but your own sense of self is not. You do and will go on.’

Petra looked at me thoughtfully. ‘What religion are you a priest of?’ she asked.

My reply was necessarily indirect. ‘Our hosts, of course, are atheists. They have a problem with theistic religions. But Buddhism was never really interested in supernatural Gods. Neo-Buddhism, which I have the honor to represent, is the first religion to base reincarnation on secure science. And so I am here today.

‘But also, of course, in other places and times,’ I added with a faint smile.

Petra looked at me indifferently but I was not fooled. When they put that blindfold on her and she contemplated her last moments on this earth, I knew that she would be reaching out in her mind to her other selves. They might not be listening, but nevertheless they were there and they would endure.


You would not be a different person if your IQ was 1.5 points lower or higher. This interval of three IQ points is roughly the test-retest consistency, one fifth of a standard deviation of 15 IQ points. Around 95% of the human population lies within plus or minus two standard deviations of the mean, so we can sort people into twenty boxes, four standard deviations times five 3-point slots per standard deviation, within each of which the people are essentially indistinguishable in intelligence.

If we do a similar exercise for the ‘Big 5’ traits of personality, then we end up with a total of 206 boxes for allocating humanity: 64 million boxes. Allocating 6 billion people uniformly to these boxes gives around 100 people per box. But most people are heaped up near the central average, so if you’re a regular kind of person you’ll be sharing your box with something like 160 people. As we’re now at seven billion, the numbers will in fact be proportionally greater.

These are your psychological twins. One day, when everyone is psychometrically tested and publicly posts their results to the future Facebook, a simple search will suffice to put you in touch with your psychological clones.

Greg Egan explored similar ideas in his short story The Walk in the collection Axiomatic.