11

The Ship of Theseus

Some conflicting (but not wholly unreasonable) theories of identity through time

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.

Plutarch (Vita Thesei 22-23)

Simple version:

Theseus’ ship carries a complete supply of replacement parts as cargo. During the long voyage from Athens to Crete, he replaces each component of his ship (planks, nails, etc.) with a new one from the cache in the hold. After each is installed, Theseus tosses the old one over the side. In this way, the ship is systematically and completely rebuilt prior to his arrival in Crete.

Question: Does Theseus arrive back in Crete on the same ship as the one on which he left?

A = the ship on which Theseus started his voyage

B = the ship on which Theseus finished his voyage

¿A = B?

ATHENS

start

Voyage

(passage of time)

CRETE

finish

Complex version:

Same replacement-at-sea scenario as above except Theseus is being followed by Scavenger, a crafty fellow who picks up all of the pieces thrown overboard and uses them to build his own boat. Scavenger arrives in Crete in a ship composed of precisely those parts that composed the ship in which Theseus left. He docks his boat next to Theseus' boat. Now we have:

A = the ship Theseus started his voyage on

B = the ship Theseus finished his voyage on

C = the ship Scavenger finished his voyage on

Problem: what is the identity / non-identity relation of A to B & C?

¿ A = B ?

¿ A = C ?

¿ Both A = B & A = C ?

¿ Neither A = B nor A = C ?