My Own Solution to Theseus’ Ship

I’m sure that someone, somewhere, already came up with this solution, but here it is in my own words.

Question: “If Theseus’ ship requires repair, and a plank is replaced, then is it still his ship? If we are inclined to say ‘yes’, then let us repair this ship so often, that in the end, every single part of the ship has been replaced, so that none of the original ‘ship’ is still present. Is it still his ship?”

What happens during the replacement of any plank is key. We somehow say that the ship we have after replacing one plank is still his ship, because most parts of the ship still belong to him (which is given to us by the assumption) However instead of treating the possessiveness of the ship by proportion of the parts belonging to Theseus, (or state that it is not his ship at all), let us think about what happens when a plank is replaced.

When a plank is replaced, the ownership of the old plank, so to speak, is transferred onto the new one. After all, when we talk about any one part of the repaired ship, by transitivity, any one subset of a set belonging to Theseus, will also belong to Theseus.

And this is the key: When the ship is first repaired, we immediately realize that this new part now belongs to Theseus/Theseus’ ship. Hence, not only is the ship his, but all its constituent parts. This is startling. If we were to recursively replace every part of the ship, it will still be his, because the moment of every piece is replaced, the ownership is transferred instantly, making the last replacement a simple “reidentification” step.

This is why building a new ship automatically constitutes his ship, even if it seems bizarre. If we were to take his entire ship apart, and build a ship out of its parts, then may not be the same ship, but it is still Theseus’ ship: During the deconstruction phase, we arrive at a point whereby the ship “disappears”, as its physical constituents are only wooden planks, pieces, etc. and not a ship. They still belong to Theseus, but his ship is gone. In other words, at some point, we cannot reidentify new objects as a ship, hence why the rebuilt ship may be his (because all the parts belong to him) but it is not the same one.

What if we would detach a specific part of the ship, so that we still identify the ship as his, while the new part forms a “new” ship, that we can also identify? In this very special case, if both parts are identified as his ship, then repairing it (i.e. adding parts one by one, reidentifying it as his ship), we get the bizarre notion that out of one ship, we now have two, both belonging to him!

This is only possible if there is no single one shape, or part, that identifies the object as a ship, which I would argue does not exist. Either way: Reidentification of the object in question allows objects to change physically, while retaining an identity, thus changing the identity of the added parts to the ship.