The Ship of Theseus is a replacement paradox also known as Theseus's paradox.

According to Greek legend as reported by Plutarch,

"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 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."

There is also an additional question: if the replaced parts were stored in a warehouse and later used to reconstruct the ship, which—if either—would be the original ship of Theseus?

A modern embellishment

If Theseus paid a nontransferable fee, allowed to be used for only one ship, for the privilege of docking in a particular harbor, would he violate the non-transferability of his license if repeated replacement of boards eventually had the result described above?

George Washington's axe

A similar story is told about George Washington's axe, with which the young George Washington is supposed, in an apocryphal story, to have cut down his father's cherry tree. The axe is supposedly on display in an (unverifiable) American museum, although, having had both its handle and its head replaced several times, no part of the original axe remains.

The Tin Woodman's story

L. Frank Baum played with this paradox in The Wonderful Wizard of Oz. At one point the book tells the story of a lumberjack named Nick Chopper. One day a witch cast a spell on Nick's axe, and it cut off his leg. He proceeded to get the leg replaced with a tin prosthetic leg, only to have the enchanted axe chop off the other. He then replaced that missing leg with a tin prosthetic, only to have the process repeat itself with both his arms, torso, and eventually head. By the end of the ordeal, Nick was entirely made out of tin replacements, yet still living, and had become the Tin Man. In a later book, the paradox is further satirized when the Tin Man meets his old flesh body parts, which have been re-assembled with magic glue. Nick's former girlfriend is not sure whom to love.

Airplanes

By FAA regulation, each airplane has a tail number to identify it. If the plane crashes and only the left rudder remains, the aircraft can be rebuilt and remains the same airplane. If the left rudder is then replaced, it is still the original airplane.

Automobiles

A similar situation exists with automobile Vehicle Identification Numbers. Each automobile is identified with a unique number inscribed in various locations, and this number is associated with the vehicle's legal registration. However, any number of parts can be replaced and it will still be considered to be the same vehicle. In some cases, an antique or desirable wrecked vehicle will be "repaired" by having its identification markers transferred to a new vehicle.

This is especially common in the cases of vehicles like the Volkswagen Beetle or Mini which remained in production for many years and have become collectable. Although such alteration was looked down on by enthusiasts and governments, at least one American company specialized in importing Beetles from Mexico to be resold as antique (and thus street- and emissions-legal) models.

On April 7, 2005, Boyd Coddington , famed hot rod designer and star of the American Discovery Channel television program American Hot Rod, pleaded guilty of perpetrating this type of fraud. Coddington's hot rods had been registered as antique automobiles in order to avoid emissions and tax liabilities. However, many of the vehicles no longer contained any parts from the original cars, and some were entirely unrelated to their supposed donor vehicles.

Computing / Digital rights management

However, unlike automobiles that can have parts and still be considered the same vehicle, Microsoft's Next-Generation Secure Computing Base considers a computer with a replaced processor (and consequentially a new Fritz-chip), to be a different computer. Consequently, it will charge for a second licence on the "new" computer.

See also

• identity and change