Note: the plausibility of the medical procedure described below is not important to this thought experiment. Your intuition about what you would experience, were it to happen, and how that intuition shapes your beliefs about your “self” are important.

You are diagnosed with an aggressive cancer that will certainly kill you in a matter of days. This is unwelcome news, but fortunately for you, you live in a time when scientific progress has made possible the once unimaginable: the exact state of your nervous system can be captured in a snapshot, and transferred to a new, lab-grown, synthetic body. Needless to say, the human lifespan has increased dramatically, and most people make the switch to a new, eternally-youthful body once their health begins to decline. Dying friends and family members who went to the renewal clinic returned in perfect health and good spirits, in lab-grown bodies made in the image of their owners in their primes.

After considering your options, you find yourself lying in a hospital bed, your medical team preparing you for the renewal procedure. Your told, “We’ll give you an anesthetic, and slowly lower your core temperature. Once your heart stops beating, the transfer will take place. In a few hours you’ll wake up in your new, healthy body”. You give the thumbs up, nervous, but not terrified, and count down from 10.

You wake up, and recall your most recent memories: lying in a bed like the one you’re in now, listening to your doctors explain the procedure to you. You notice that you feel good, better than you have in years. You sit up and walk to the bathroom, where you’re delighted to see the face of your younger self grinning back at you. A nurse comes into your room and asks you a series of questions to determine if your memory and mental faculties are intact. You check all the boxes, and you’re taken to a room where you can change back into your clothes. On your way out of the facility you’re asked to sign out and rate your experience: 5 stars.

Let’s rewind a bit and consider another outcome. You give the thumbs up, nervous, but not terrified, and count down from 10.

You wake up, and recall your most recent memories: lying in a bed like the one you’re in now, listening to your doctors explain the procedure to you. You feel cold and groggy, and upon opening your eyes, know for certain you’re in your old body, and you’re still dying. The fear of death, which was absent before, rushes in with full force. With an effort, you get to your feet and rush out of the room and across the hall to the room you know your new body was meant to occupy. You find it already abandoned, but the chart left there reveals to you that the transfer was a success and the new you has checked out of the facility. But it isn’t really you, can’t be you, because you’re here, experiencing this. And you’re terrified, because you’re dying, and this time, there is nothing to alleviate your existential fear.

These two outcomes are not mutually exclusive. At the moment the transfer completes, there are two unconscious versions of you with the identical mental state. Either one or both can regain consciousness, and then begin diverging from this mental state by having new experiences and forming new memories. The question is which body you actually wake up in, if you wake up at all. If the old body never comes to, is your intuition that you wake up in the new body? If both bodies regain consciousness, is your intuition that you wake up in the old body?

For me, imagining this fork in the road of the continuity of self, where two persons with all of my memories exist for a moment in time (and then immediately begin to form new memories and diverge), and trying to imagine which one I would inhabit as a “self”, motivates for me that something about my intuitive sense of the self is an illusion.