The Ship of Theseus and the nature of identity

HostI was looking at an old hammer in my shed the other day. It belonged to my grandfather. Over the years, the wooden handle cracked, so I put on a new one. Then the heavy metal head got too rusty to use, so I swapped that out for a new one too. It still feels like my grandfather’s hammer when I hold it, even though every single part of it has been bought from a store in the last five years. It makes me wonder what actually makes the hammer what it is.

HostThis is an old puzzle that people have been chewing on for thousands of years. They usually tell it with a ship instead of a hammer. The story goes that a hero named Theseus had a great ship. After his many trips, the ship stayed in the harbor. Over time, the wood started to rot. To keep the ship looking good, the people replaced the old planks with new, strong ones. One by one, the old wood went out and the new wood went in. Eventually, there wasn't a single piece of the first ship left. Every nail, every board, and every sail was new.

HostIf you look at that ship in the dock, is it still the ship of Theseus? On one hand, you might say yes. It stayed in the same spot, it looks the same, and people still call it by the same name. But on the other hand, the actual stuff that made up the ship is gone. If the ship is just a pile of wood, and all that wood is new, it seems like it must be a new ship. This is where we start to see a split between the stuff something is made of and the shape or role it has in the world.

HostNow, it gets a bit more messy. Think about a guy who decides to follow those workers around. Every time they throw away an old, rotten plank, he picks it up. He takes it home and dries it out. Once he has all the old pieces, he puts them back together in his own yard. Now we have two ships. There's the one in the dock with all the new parts, and there's the one in the yard made of the original wood. If Theseus came back and wanted his boat, which one would he take?

HostIf we say the boat in the yard is the real one because it has the original wood, then the boat in the dock must have stopped being the ship of Theseus at some point. But when? Did it happen when the first plank was swapped? Or when half the wood was gone? It feels strange to say that changing one small board creates a brand new ship. But it also feels strange to say a ship made of old, discarded scraps isn't the ship those scraps first belonged to.

HostWe can take this same idea and point it at ourselves. You might feel like the same person you were ten years ago, but your body tells a different story. You're a living ship of Theseus. Your skin is constantly falling off and growing back. The red cells in your blood only live for about four months before they're replaced. Even your bones, which seem so solid, are mostly swapped out over a decade. Most of the tiny bits that make up your body right now weren't there when you were a child. They came from the water you drank and the food you ate last year.

HostThis means that if being you depends on the actual meat and bone you're made of, you're not the same person who started school or learned to ride a bike. That person is mostly gone. But we don't usually feel that way. We feel like there's a thread that runs through our whole life. We think of our self as the pattern, not the stuff. It's a bit like a song. You can play a song on a piano or a loud drum set. The sounds are different, but the song is the same because the notes are in the same order.

HostBut even our patterns change. Our memories fade or change over time. The way we think and the things we love when we're five are very different from when we're fifty. If the wood changes and the shape of the boat changes too, what's left to hold it all together? Some people think that the name and the story are what matter. We're the same person because we and the people around us keep telling the same story about who we are.

HostMy name is Miles, and I find it odd that we spend so much time trying to keep things the same when the world is always moving. There's an old thought that you can never step into the same river twice. The second time you dip your toe in, the water has moved on and you have changed too. We like to think of ourselves as solid rocks, but we might be more like the waves in the ocean. The wave moves across the water, but the water itself is just moving up and down. The wave is a movement, not a thing.

HostA candle flame is always pulling in new wax and new air to keep burning. The light we see looks like one steady thing, but it's actually a fast flow of energy that never sits still for even a second. Your skin cells are replaced about every thirty days.