Ship of Theseus
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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."
Paraphrased, if the entire ship was replaced, piece by piece, would it be, in the end, the same ship?
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?
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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?
The philosophical point of this example is that it sometimes makes sense to regard an object's identity as the same for a particular purpose even if it might be different for some other purpose.
Heraclitus
The Greek philosopher Heraclitus took an opposing view of metaphysical identity by stating "No man can cross the same river twice, because neither the man nor the river are the same."
Aristotle's causes
According to the philosophical system of Aristotle and his followers, there are four causes or reasons that describes a thing; these causes can be analyzed to get to a solution to the paradox. The Formal Cause or form is the design of a thing, while the Material Cause is the matter that the thing is made of. The Ship of Theseus, in a limited sense, could be described as the same ship, because the formal cause, or design, does not change, even though the matter used to construct it may vary with time. In the same manner, a river has the same formal cause, although the material cause (the particular water in it) changes with time. Another of Aristotole's causes is the end or Final Cause, which is the intended purpose of a thing. The Ship of Theseus would have the same end, that is, transporting Theseus, even though its material cause would change with time. The Efficient Cause is how and by whom a thing is made, for example, how artisans fabricate and assemble something; in the case of the Ship of Theseus, the workers who built the ship in the first place could have used the same tools and techniques to replace the planks in the ship.
Examples
Automobiles
Every automobile is identified with a unique Vehicle Identification Number. This number is inscribed on the car in various locations, and is associated with the vehicle's legal registration. However, any number of parts can be replaced - including the 'VIN plate' bearing the number and it will still be legally 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. A vehicle which has has its identity switched in this manner is known as a re-VIN.
R. Daneel Olivaw
In the book Foundation and Earth, the ancient robot R. Daneel Olivaw says that over the thousands of years of his existence, every part of him has been replaced several times, including his brain, which he has carefully redesigned six times and constructed a new brain containing the positronic pathways containing his current memories and skills, along with free space for him to learn more and continue to operate for longer.