The Ship of Theseus

Recently I read a reference to the Theseus Paradox. I always find such items fascinating, such as the Grandfather Paradox which is used to argue inconsistencies in time travel: if one travels back in time and kills one’s grandfather, so that one never can be, how did one travel back in time to kill him? 

The Theseus Paradox, also known as The Ship of Theseus, deals with the issue of identity: whether an object that has had all of its components replaced remains fundamentally the same object. 

Some comments in brief: 

• The first recording of the paradox is by the ancient Greek historian Plutarch, but it may have existed before him. Theseus was an ancient hero of Greek mythology, the supposed founder of Athens, and the conqueror of various monsters, including the Minotaur. He also won several naval battles. 

• According to Plutarch: 

The ship wherein Theseus and the youth of Athens returned 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 place, 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. 

• Theseus’s shop remained in the harbour for centuries. As parts and boards rotted, they were replaced. If all the parts and timber are replaced, is it still the same ship? If not. at what point does it cease being the original ship? When one board is replaced? When half the ship is changed? 

• 17th century philosopher Thomas Hobbes took the paradox one step further: if a person gathers up all the old boards and timber that was replaced and reconstitutes them into the ship again, is that the original ship? 

• But this is not just an abstract mental exercise. Just as the ship is altered, so people alter as they age, grow in knowledge, change their situations and so on. There is no fixed, immutable identity for each person. Harvard psychologist Daniel Gilbert put it well: “Human beings are works in progress that mistakenly think they’re finished.” The Theseus paradox reminds us that the identities of us all are not fixed and solid, like statues, but malleable and ever-changing. 

BTW: 

There’s an old joke that a person shows another man an axe and tells him that it is the axe that George Washington used to chop down the cherry tree, although the handle has been replaced four times and the head twice.