The Ship of Theseus

Now this body of mine consists completely of the flesh of another person. Do I truly have a body now? Or do I have no body? If I think I have a body, what I have is completely a body of another; if I think I have no body, now I actually do have a body.” Thinking this way, he made his heart bewildered, and he was like a mad man.

— Dà zhìdù lùn

As the Greek story goes, Theseus sailed from Athens to Crete to rid the world of the minotaur hidden in the labyrinth. When he returned in success, the people of Athens decided to honor Apollo for the result by sailing the boat Theseus used every year to the island of Delos, the birthplace of the god. The Athenians did not intend this tribute to be short-lived. Over centuries, they maintained the boat to keep it seaworthy, replacing rotted wood or faulty parts when needed.

In our previous article, we examined the immortal jellyfish. This being features the ability to turn back the clock under potentially fatal circumstances, as its adult cells transform into the polyp phase of the jellyfish life cycle. From this point, it grows old again. Thinking about this incredible phenomenon, we asked whether this jellyfish remained the same individual as it continued to morph from adult to child to adult to child.

Our discovery of the jellyfish superpowers might represent a uniquely modern, scientific illustration of this philosophical query, but the question is hardly new. In Life of Theseus, Plutarch – historian, philosopher, and color-commentator-extraordinaire of Ancient Greece – pondered what we today call the Ship of Theseus:

“The ship wherein Theseus and the youth of Athens returned from Crete had thirty oars, and was preserved by the Athenians down even to the time of Demetrius Phalereus, for they took away the old planks as they decayed, putting in new and stronger timber in their places, insomuch that this ship became a standing example among the philosophers, for the logical question of things that grow; one side holding that the ship remained the same, and the other contending that it was not the same.”

This thought experiment became one of the most famous philosophical examinations, touching on concepts of identity, material constitution, and the mind-body connection.

If an object made of wooden pieces begins to deteriorate, one can replace the parts with fresh lumber. The form remains the same but the material pieces have changed. If we substitute one part, is it the same ship? If, in a few decades, new timber now constitutes two-thirds of the ship, is it still the same craft? If, after many years, the entire vessel is now composed of new parts, is the Ship of Theseus still this Ship of Theseus?

If one were to watch the original ship in the harbor 24 hours a day, the form would never disappear. Mechanics might hop on board with shiny components, but the vessel would continually bob in the waves. In this case, one could argue it never ceases to be the Ship of Theseus.

Conversely, if all the atoms that comprised the original ship have been replaced, can we say it’s still the original? One might contend that this situation occurs in the human body. Our bodies replace 330 billion cells every day. Based on these numbers, the mass in our forms is replaced every 80 to 100 days. Some cells last longer than others, so some scientists have asserted we can view new constituent atoms in a human body approximately every seven years. Most people would view themselves as the same individual, despite new “parts,” so perhaps we can view the Ship of Theseus the same way. However, do we view ourselves as the same individual because we buy this side of the argument or do our minds, consciousnesses, or souls provide the string to remain a constant form? If you buy this argument, does the Ship of Theseus have a spirit that binds it through new parts?

This paradox defies an easy solution.

A millennium after Plutarch, philosopher Thomas Hobbes provided another wrinkle.

In “Of Identity and Difference,” Hobbes wrote:

“For if that Ship of Theseus were, after all the Planks were changed, the same Numerical Ship it was at the beginning; and if some Man had kept the Old Planks as they were taken out, and by putting them afterward together in the same order, had again made a Ship of them, this, without doubt, had also been the same Numerical Ship with that which was at the beginnings and so there would have been two Ships Numerically the same, which is absurd… But we must consider by what name anything is called when we inquire concerning the Identity of it… so that a Ship, which signifies Matter so figured, will be the same, as long as the Matter remains the same; but if no part of the Matter is the same, then it is Numerically another Ship; and if part of the Matter remains, and part is changed, then the Ship will be partly the same, and partly not the same.”

Hobbes envisions someone collecting the rotting wood as it’s replaced. When the entire ship is composed of new parts, someone takes the old pieces and makes a new ship with the original blocks. Is this second ship actually the original? Do two Ships of Theseus now exist? Hobbes asserts the idea absurd that both ships be considered the Ship of Theseus, but which should now be considered the proper item?

In the Hobbes quote above, he refers to “numerical” identity, as opposed to “descriptive” identity. Descriptive identity refers to two objects that contain all the same traits. Numerical identity pertains to a singularity. If two terms – for example, “Didier Drogba” and “Greatest of All Time” – refer to the same individual, they are numerically identical. Two red cars of the same make, model, and year are descriptively identical but not numerically identical.

An in-depth examination of this quandary could balloon. Dozens of famous philosophers have tackled the topic, and dozens of proposed solutions exist. Many arrive at the idea that material constitution is not identity, that form and purpose matter most. Others believe the paradox arises in the human mind and not the material world (i.e. no paradox of matter exists; we “see” the problem). Still, others lean on the problem of language: is the ship a thing or an organizational schematic? Some philosophers divide the parts of the ship or ships into “time-slices,” arguing that two ships existed at all times.

The notion has taken many forms over the millennia, from Lincoln’s axe to Jeannot’s knife to Locke’s sock. Another ancient example arrives from a Chinese Buddhist text via Sanskrit. In this tale, two demons quarrel over a corpse. One demon stole the body from the other and absconded. When the second demon caught up with the thief, they encountered a man and decided to ask him to settle ownership. The thieving demon used a rhetorical trick, asking the man who “arrived” with the body. The man had no choice but to answer with what he witnessed. The demon who originally carried the corpse became angered, and it began tearing the man’s body parts off. The thieving demon took corresponding parts from the dead body and replaced them on the man. Eventually, his entire body became composed of parts of the corpse, while the demon assembled a new body out of the living man’s parts. The demons then devoured the new body and left the man to ponder his new identity.

The Buddhist framing adds another level to the paradox. As the man goes mad, wondering if he is still himself or could still be termed to have a body, he meets monks who counsel him: “This man has learned by himself that there is no I. He can easily obtain deliverance. Your body, from the very beginning, has always had no I, not just now.” The lack of self is a large tenet of many strands of Buddhism, so these monks did not sympathize with the demon-eaten man. Instead, this experience simply illuminated that he was not the Ship of Theseus before or after being munched by a demon. An extremely comforting take.

These exercises might seem more fit for the potential arhat than one examining the natural world. However, the paradox has tangible applications. In addition to the immortal jellyfish and future issues involving artificial intelligence or its melding with human brains, a real Ship of Theseus sits in Boston Harbor. The USS Constitution launched in 1797 and is now the world’s oldest commissioned warship still afloat. Since then, it has undergone numerous renovations. Is this ship still the original? As the Antique and Classic Boat Society puts it: “The USS Constitution has essentially none of its original wood but we believe no one would consider it a replica. It is Old Ironsides.”

More like New Ironsides.

Perhaps the most fun application of the Ship of Theseus belongs to the internet. Every article on Wikipedia contains a history of any edits, as both a categorical feature and a safeguard against erroneous changes. Many on the net point out that the article on the Ship of Theseus now contains no phrases from the original! The article has become a meta-example of the paradox itself.

A quick review of the current and original post shows that the original Plutarch quote appears in both versions, but we won’t let that ruin the sentiment.