Ship of Theseus

The Ship of Theseus is a philosophical thought experiment that explores the issue of identity. In essence, the experiment asks if an object has some or all of its individual parts replaced, is it still fundamentally the same object? The issue was first raised by the Greek philosopher Plutarch, who used the preserved ship of the mythical king Theseus as an example. In the experiment, which is also called Theseus’s paradox, Plutarch asked if all the individual wooden planks of the ship were replaced over time when they rotted, would the ship still be the original ship of Theseus? The question has intrigued philosophers for centuries and has been adapted to create related thought experiments as well. It has also been used to examine human identity, specifically asking if our bodies’ cells replace themselves over a period of time, are we the same person we were days, weeks, or years ago?

Background

According to Greek mythology, Theseus was a legendary king of Athens who was the son of either the god Poseidon or the Athenian King Aegeus. Theseus made a name for himself with his heroic deeds, killing a series of monsters and human enemies, usually by the same methods his enemies used to kill their victims. In his most famous adventure, Theseus took on the feared Minotaur, a half-man, half-bull creature on the island of Crete. The king of Crete had been demanding an annual sacrifice of fourteen Athenians—seven young men and an equal number of young women—to feed the Minotaur. Theseus volunteered to go to Crete and fight the beast. Using his ingenuity and bravery, Theseus killed the Minotaur and returned to Athens a hero.

Although Theseus existed only in myth, he was still the subject of a biography written by the Greek philosopher and historian Plutarch. In Plutarch’s Parallel Lives, which was written in the late first century Common Era (CE), he claims the Athenians honored Theseus upon his return from Crete by turning his ship into a memorial. The ship was said to have remained in the harbor of Athens for centuries, at least until about the third century Before the Common Era (BCE).

Overview

Plutarch wrote that the ship had thirty oars and was carefully preserved by the Athenians. He said that as the ship’s old wooden planks decayed, they were replaced with new ones made of stronger timber. Over time, all the wooden planks of the original ship must have decayed and been replaced. Plutarch then raises the idea that some philosophers will argue that the ship is no longer Theseus’s original ship, while others will argue that it is.

Plutarch was not the first Greek philosopher to explore the idea of identity in the face of constant change. Heraclitus, who lived about 500 BCE, said that no person could step in the same river twice, as the constant movement of the river means a person would step into different water each time. Plutarch’s idea asks, is the ship of Theseus the same ship the hero sailed upon, even if all its individual parts have been replaced? If not, then at what point does the ship stop being Theseus’s ship and become something else? Does this happen when the first plank is replaced, when half the planks have been replaced, or when all the planks have been swapped out?

The question is purely rhetorical and does not have a correct answer. Some philosophers have argued that the ship becomes a different ship the moment one of the planks has been changed. Because it has a new element, it is no longer the original object but a different one. Others claim that if at least one original plank remains, the ship is still the same one. Another answer is that even if all the planks have been replaced, the ship is still the original. This view holds that the ship retains the form, or essence, of the original and would still be considered the true ship of Theseus. It can also be argued that the ideal the ship represents has not changed, despite its new planks, and, therefore, it maintains its original status. A more theoretical idea suggests that all objects exist in a fourth dimension: time. The ship and the rest of the universe move through the fourth dimension in a time slice we call the present. Using this idea, the ship exists from the past into the future, with humans only able to experience it in the present.

The theory of the Ship of Theseus was further explored by Enlightenment thinkers. In the seventeenth century, British philosopher Thomas Hobbes added his own take to Plutarch’s question. Hobbes wondered what would happen if someone found all the discarded and decayed old planks of Theseus’s ship and built an entirely new one out of the pieces. Which ship would be the original ship? Would it be the ship in the harbor constructed with new timber or the ship made out of the pieces of the old ship?

Another seventeenth-century English philosopher, John Locke, came up with his own variation on Theseus’s paradox. Locke asked what would happen if his sock had a hole that was patched up. Would that sock be his original sock? Suppose a second patch was added, then a third, and so forth, until the entire sock was made of patches. Would the sock still be his sock? Still another variation of the question involves an ax used by George Washington. If the ax has had both its handle and head replaced, is it still Washington’s ax?

Philosophers have often used the Ship of Theseus question to examine human identity and change. People tend to see themselves as reaching a point of maturity in life where they are complete, at least in a physical sense. However, the human body is constantly changing. Cells die and are replaced at a regular pace. Some cells, like those in the intestines, last only a few days. Skin cells die and are replaced every few weeks, and red blood cells every few months, but brain cells can last an entire lifetime. So, the question is, what makes a person a person? Is the person one was ten years ago the same person the individual is today, even if most of the body’s cells are completely new ones? Is psychological continuity more important than physical? Again, there is no right answer, and the same solutions used for Theseus’s paradox apply here as well.

In the twenty-first century, scientists, philosophers, and people in many other fields continue to apply the Ship of Theseus theory. Specialists apply the theory to issues of human identity, to the proliferation of Artificial Intelligence, within the broader concepts of technological advancement, to fields of medicine such as organ transplants and regenerative medicine, and to neuroscience or the study of the brain and its thoughts and memories. The Ship of Theseus also continued to appear in literature and popular culture references, including the television show, "WandaVision," which is part of the Marvel Universe.

Bibliography

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