This is another oldie-but-a-goodie: the Ship of Theseus. It has been floating around since the ancient Greeks thought it up, and it’s only gotten more interesting as technology and science have changed our understanding of how the world works. The story goes something like this: Once upon a time there was a guy named Theseus, and he had a ship. It was a big, grand ship, with lots of parts, including sails, boards, nails, and crew members. This ship sailed all around the world, trading goods all over the place. Little by little, the wear and tear of the ocean weather meant that the boat needed repairs: a new board to replace a rotten one, a new sail after an old one tore, and new crew members as the others died of scurvy or pirates. Eventually, even Theseus himself died. Many, many years after the boat’s original owner had passed away, a crew member decided to get a new oar made. Unbeknownst to him, that oar was the last remaining original component of the ship. As he tossed that oar aside, it became true that no physical part- not a nail, not a stitch of cloth, not a single deckhand- of the original ship of Theseus was in this current ship.
Now, it just so happened that Poseidon had taken an interest in this ship. He thought it was a very nice ship, with great craftsmanship and power. He was so intrigued by this ship, in fact, that he took it upon himself to gather every last discarded part- every board washed out to sea, every fleck of paint, every dead body- and assemble those parts back into the shape of a ship, and put it on his bookshelf. It was a gross, disgusting ship, all rotted out and smelly, but still, it raised an interesting question- which ship was the real ship of Theseus- the one with all the original parts, or the one still sailing around? The ship currently sailing on the ocean had no physical part in common with the ship that Theseus originally built. Nothing physical was there to make it the real ship of Theseus. On the other hand, the ship on Poseidon’s shelf had every single part of the ship Theseus made.
Neither answer seems to work quite right. If you say that the ship Poseidon has is the real ship, then when, exactly, did the ship sailing around stop being the real ship of Theseus? Was it when it lost the very first fleck of paint? Or was it later, when the last oar was thrown away? Somewhere in the middle? Maybe it changed when Poseidon made his ship. But how could his project effect the ship sailing away on the surface? But if you say that the ship sailing around is the real ship of Theseus, then what makes it so? There is no physical part that ties that ship to the ship that Theseus made. Is there some intangible thing that makes the ship with all new parts ‘real’?
This becomes much more distressing when you realize that this doesn’t just apply to ships, or even just to machines. You, dear reader, are a Ship of Theseus. So am I. Every day, we eat new food, and we poop old food out. Little by little, every part of us is replaced with new material. In fact, scientists estimate that every five years, every single atom in your body is changed over. Chances are, none of the atoms in your body were yours five years ago. And it’s not just our bodies. How many character traits, opinions, and memories do you share with the person you were when you were twelve? When you were a baby? Here, it ties back to the Paradox of the Heap. How many things about you can change before you aren’t ‘you’ anymore?
I guess we should just be thankful that no one is going around gathering up our discarded matter and making new people out of it. Otherwise, this vaguely worrying thought experiment might be a lot more pressing!