Novy Metaphysics SP08

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)

Two modern versions

Theseus completely rebuilds his ship, replacing each of the component parts while at sea. Theseus’s ship carries a complete supply of replacement parts as cargo in its hold. Once out to sea, Theseus systematically replaces each component of the ship with a new one from the cache in the hold.

Simple modern version:

Theseus completely rebuilds his ship, replaces all the parts, and throws the old planks, etc. overboard. Question: Does Theseus arrive back in at the dock on the same ship as the one on which he left?

Let A = the ship on which Theseus started his voyage

Let B = the ship on which Theseus finished his voyage

So, Does A = B?

Complex modern version:

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

Let C = the ship the on which The Scavenger finished his voyage.

So, which is it?

A = B	Both A = B & A = C
A = C	Neither A = B nor A = C

If the ship of Theseus were continually repaired by the replacing of all the old planks with new, then – according to the Athenian philosophers—the latter ship would be numerically identical with the original. But if some man kept the old planks as they were taken out and were to assemble a ship of them, then this ship [containing all the original parts of the earlier ship] would, also, without doubt be numerically identical with the original. And so there would be two ships, existing at the same time [in different places,] both of which would be numerically identical with the original. But this latter verdict is absurd.

Hobbes (De Corpore, [97],Part II, chapter, §7)