Ship of Theseus Paradox

by | Jun 27, 2018


As addressed in a prior IFOD, the cells in our bodies are constantly being replaced. In ten years you’ll be an entirely different organism in terms of your cells (other than a few types of cells – notably the cerebral cortex – which are not replaced). So, are we the same person still even though we have all new cells? This brings up a classic philosophical paradox called the “Ship of Theseus” and concerns the question of what makes something the same even though it changes over time?

The basic statement of the problem as posited by the philosopher Plutarch is: Imagine that Theseus (founder/king of Athens) is sailing back from a military conquest. Along the way home, one-by-one, all the wood planks of the ship are thrown overboard and replaced with newer pieces of wood. At the end of the voyage, the ship is made entirely of new wood. The question: is the ship they returned on the same as the one they left on? It looks the same and is the same in all regards, but it is made of different wood. Additionally, Theseus never left the ship. So, is it “his” ship or not? Would your answer change if the entire ship was made of new wood except for one plank? Except for ten planks?

Centuries later Thomas Hobbes added his own twist to the Ship of Theseus question: assume the same facts as the original formulation but now assume that as the old pieces of wood were thrown overboard that a scavenger traveling behind Theseus gathered the discarded wood and re-built the ship from the discarded wood. Now there are two ships: the one built of new wood and the second ship rebuilt from the original parts? Which ship is Theseus’ ship? The one Theseus stayed on or the one that the scavenger re-built from the original parts?

These fact patterns hurt my brain. What’s the answer? There isn’t one, but there are two primary theories which are used to think about this problem:

1. Merelogical Theory of Identity (MTI). This theory states that the identity of something is dependent on the identity of the component parts. If the parts are different, then the thing is different. Under this theory you look at something at Time One (T1) and at Time Two (T2); if the thing at T2 has different parts than the thing at T1, then the two things are different. Even if they look the same.

2. Spatiotemporal continuity theory (STC). This theory states than an object maintains its identity as long as the change is gradual and the object has a continuous path in space-time.

Both are compelling theories, but both have problems and the “right” answer has not been settled. An issue with MTI is that Theseus stays on “his” ship during the entire voyage, yet MTI would conclude that he sailed into port on a different ship. MTI also suggests that every decade or so we humans are a different person because we are made up of almost entirely new cells. This doesn’t seem to be the right answer.

STC has its problems as well.  Think of the fact situation where Theseus’ ship is taken apart, shipped in separate crates on separate ships to a new location and then reassembled. Under STC, it’s a different ship. because it didn’t retain it’s identity through space-time. But we know its still the original ship – just reassembled.

Another commonly discussed example that disproves STI from being the answer is: “Suppose the ship (A) is in a museum, and a clever ring of thieves is trying to steal the ship by removing its pieces one at a time and then reassembling them. Each day, the thieves remove another piece, and replace it with a look-alike. When they have removed all the original pieces, we are left with this situation. There is a ship, B, that is in the museum (made of all new materials), and there is a ship, C, in the possession of the thieves (the original pieces of A now reassembled). Which ship is A (Theseus’s original ship)? Surely not B—it’s just a copy of A, left behind in the museum by the crooks to cover up their crime. It is C that will interest the antique dealer who is interested in buying A, the original ship.” (Quote from Marc Cohen, University of Washington)

The Ship of Theseus paradox can be applied to a lot of situations. For example, it can be applied to a company. Assume a company is founded and over the next 75 years every single employee turns over. Is it the same company? What if it changes it’s office location? Changes its name? Has 100% new shareholders? Is bought by a private equity company? Acquires five other companies, all of which are much larger than the original company? Moves industries (think IBM moving from a computer manufacturer to a consulting firm). Under each of the previous questions, is it the same company?

Here’s a great video from a professor of philosophy at the University of Georgia explaining and digging into the Ship of Theseus problem (very interesting):

Like paradoxes? Here’s a few other IFODs on various paradoxes:

The Interesting Number Paradox (The First IFOD)

Decision-Making Under Uncertainty: The Ellsberg Paradox

The Paradox of Skill

Braess Paradox


  1. From my view, which does not matter much, the answer is perspective. It is what ever the person or organization with the appropriate authority wants it to be. If you are the board of directors of IBM and have the authority to name the company, and your perspective is that IBM has value. You can choose IBM as the name. That might get challenged the courts and in that case it will get decided by an “unbiased judge” who has the best interest of the people as his guiding principle. Ya Think!

  2. Really interesting. We constantly deal with this issue in historic preservation of buildings while trying to upgrade and adapt the structures to modern uses. Reminds me of George Washington’s hatchet in the Smithsonian …it’s had five handles and 2 heads.


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