Identity Change

The Ship of Theseus

At its most basic level, identity change suggests that at one time (in the past) an object had a specific set of properties that made it one thing, but it is now (in the present) a different object with a completely new set of properties that define it. The most familiar philosophical example of identity change is the Ship of Theseus, which comes to us in print via Plutarch. The Ship of Theseus refers to a scenario in which, as the planks of the original ship decayed, the Athenians replaced them one by one. Over time, there were eventually no planks remaining from the original ship. So, the dilemma emerges: Is the ship displayed by the Athenians the true Ship of Theseus, or is it a new ship altogether? To push this dilemma even further, one variation of this tale sets the vessel in motion. As the planks of the ship at sea (A) rot, they are thrown overboard and replaced with identical replacement planks. Following the ship, however, is a scavenger boat (B), which picks up the planks thrown overboard and uses them to replace its own planks. At the time of its arrival, the original ship (A) has discarded and replaced all of its original parts, while the scavenger boat (B) has taken up the discarded parts and reconstituted the ship with the original parts thrown overboard by the original ship (A). Which of the two ships, then, is the real Ship of Theseus? Is it the ship on which Theseus was aboard throughout the voyage (A), or is it the scavenger boat that has now reconstituted all of the original parts from the original ship?

The answer to this question depends on what conception of a thing's identity one adopts. If a thing's identity is defined by its material constituents, then clearly the scavenger boat (B) is the true Ship of Theseus. However, this yields the odd result that Theseus has, at some point in the voyage, changed ships, even though Theseus obviously remained on one single boat throughout the voyage. As a result, one might opt for the alternate conclusion that the ship on which Theseus remains (A) is the true ship. This is known as the spatiotemporal continuity thesis, according to which the identity of the ship is understood in terms of the ship's underlying continuity throughout its journey from its point of departure to its destination. This thesis, likewise, yields an odd result in that it leads to the conclusion that two boats with exactly the same components—belonging to the original ship (A) at the point of departure and the reconstructed ship (B)—would be different ships in spite of their material identity.

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