To understand the classic paradox of the ship of Theseus, one must first understand what a paradox is.

Philosophical Definition

PARADOX: In philosophy, a paradox is a statement that begins with a premise that seems true; however, upon further investigation, the conclusion ends up proving that the seemingly true premise is actually false.

The first time the ship of Theseus paradox appeared in print was in the writing of the ancient Greek philosopher (and Platonist) Plutarch. Plutarch writes of Theseus (the founder-king of Athens) returning from a long voyage at sea. Throughout the voyage, all of the old, decaying planks of wood the ship was made of were thrown overboard and replaced with new, strong pieces of wood. By the time Theseus and his crew finally returned from their trip, every piece of wood that the ship was made from had been replaced. This leads to the question: Was the ship that they returned on the same ship that they left on, even though it was made of completely different pieces of wood? What if the ship still had one of the original pieces of wood in it? What if there were two pieces of wood still in the ship? Would this change one’s answer?

Another way to look at it is this:

If the ship Theseus began his journey on is A, and the ship Theseus ended his journey on is B, then does A = B?

Much later, the famous seventeenth-century philosopher Thomas Hobbes took the paradox one step further.

Now, imagine that following Theseus’s ship is a scavenger. As Theseus’s crew throws the old pieces of wood overboard, the scavenger takes them out of the water and builds his own ship. Two ships arrive at the port: one with Theseus and his crew, made out of new wood; the other, the scavenger’s ship, made entirely out of the old wood that Theseus’s crew had thrown overboard. In this scenario, which ship is Theseus’s ship?

In this scenario, let’s call the boat the scavenger arrived in the letter C.

We know that B ≠ C because two ships land in the harbor and so they clearly cannot be one and the same.

So what makes something the ship of Theseus? Is it the individual parts that the ship is made from? Is it the structure? Is it the history of the ship?

One theory, known as the mereological theory of identity (or MTI), states that the identity of something is dependent upon the identity of that thing’s component parts. This theory claims that a necessary condition of identity is that there must be a sameness of parts.

In other words, X = Y if all of the parts of X are also a part of Y and vice versa.

For example, object X is composed of certain components at the beginning of a period of time (t1). If by the end of that period of time (t2), the object (which is now Y) has the same components, then it continued to exist.

In the ship of Theseus paradox, according to MTI, A = C. This means that there are two ships. The ship Theseus began his voyage on is the exact same as the ship the scavenger comes in on (making these one ship), and then there is the ship Theseus came to port in, which was composed of new parts.

However, there is a problem with this conclusion. In this scenario, Theseus would have had to change ships in his journey because he comes to the port in B (which does not equal C). But Theseus never leaves his ship. He leaves on A, comes back on B, and was never aboard two ships (which MTI states there must be).

There might be other possible ways to solve this problem. We can abandon what MTI states altogether and instead claim that A = B. In this scenario, there are still only two ships: the ship Theseus began his journey in (A) and the ship he came back in (B) are considered one, and the scavenger’s ship is the second.

This scenario also raises problems. To say that A = B would also imply that B ≠ C and therefore A ≠ C. But one cannot feasibly say this because every part of C is a part of A and vice versa. In addition, A and B do not have any parts in common, and yet we are claiming that they are the same ship.

Another theory that can be applied to the paradox of Theseus’s ship is called spatiotemporal continuity (STC). This theory states that an object can have a continuous path in space-time, as long as the change is gradual and the shape and form are preserved. This would allow for the gradual changes that are made to the ship over time.

However, even here we see problems! What if every piece of the ship was packed in individual boxes, shipped all over the world to different locations, then shipped back, and then opened and reassembled? While numerically it may be the same ship, the object does not constantly exist as a ship-like object through space-time (note that MTI does seem to fit in this scenario).

Of course, this paradox goes beyond a problem about ships. The ship of Theseus is really about identity and what makes us the people that we are. Parts of ourselves change as the years go by, and yet we still consider ourselves to be the same person.

Is our identity the same because of our structure? If that were the case, if you were to lose a limb or even cut your hair, you wouldn’t be you anymore. Is it because of your mind and feelings? If that were the case, are you no longer yourself when you lose memories or have a change of heart? Is it because of the parts we are made up of? Our history?

The ship of Theseus and its implications about what identity is are still discussed to this day.