The Self Is a Verb

My biology teacher told us the body is always rebuilding itself. Red blood cells last about 120 days. The outer layer of skin renews every few weeks. Even bone, which seems like the most permanent part of us, gets broken down and rebuilt over years. So it sounded like, given enough time, nothing of the original you is left.

I got curious and looked it up, and it turns out that’s only mostly true. In 2005, a team led by Jonas Frisén at the Karolinska Institute in Sweden found a clever way to figure out how old human cells are. Nuclear bomb tests in the 1950s and 1960s raised the amount of carbon-14 in the air, and a cell’s DNA keeps a record of the carbon-14 level from the year the cell was made. Using that like a timestamp, Frisén estimated that the average cell in an adult might be only seven to ten years old. But the neurons in the cerebral cortex turned out to be as old as the person. That only works because the atoms in a neuron’s DNA stay put for its whole life. Not everything gets replaced, then. Still, even inside those lifelong neurons, lots of the molecules are rebuilt all the time, so the puzzle doesn’t go away.

It’s also a very old puzzle. Plutarch wrote about the ship of the hero Theseus, which the Athenians kept for generations by swapping out rotting planks for new ones. He said philosophers used it as an example and argued about whether it was still the same ship. Centuries later, Thomas Hobbes added a twist: what if someone saved all the old planks and rebuilt the ship with them? Which one is the real ship? You can try deciding for yourself in Which One Is the Ship? If I’m just my materials, then I become a slightly different person every time I lose a skin cell, and that seems wrong.

John Locke gave a different answer in 1694. He imagined the soul of a prince, carrying the prince’s memories, moving into the body of a cobbler. Locke said everyone would see that the person is now the prince, because what makes you the same person is your chain of memories and consciousness, not your body. (The personal identity primer goes into this more.) If Locke is right, swapping out cells doesn’t matter, as long as your memories stay connected.

But memories are physical too. They’re stored in the connections between neurons, and those connections depend on the same constant rebuilding. That’s why the mathematician Norbert Wiener makes the most sense to me. In his 1950 book The Human Use of Human Beings, he wrote: “We are but whirlpools in a river of ever-flowing water. We are not stuff that abides, but patterns that perpetuate themselves.” A whirlpool keeps its shape even though the water spinning through it is always new. It’s the same whirlpool because of the pattern, not the water.

I think that’s the best answer biology gives to the old question. The self isn’t a thing made of certain atoms, and it isn’t just a list of memories either. It’s a process of rebuilding that keeps its shape. Some of my neurons have been with me since I was born, a lot of the rest of me is younger than I am, and the pattern holding it all together is still, somehow, me. The self is less like a noun and more like a verb.