Suppose there is a ship that, in the year 1800, sails out to sea. This ship is made of 100 boards, a sail, and a mast, and is called "The Ship of Theseus." A year goes by with the ship out to sea, when a single board of the ship's original 100 is replaced by brand new wood. The old board is hucked out to sea; the new one nailed neatly into place. And so the ship sails on...Another year goes by with the ship out to sea, when another single board of the ship's original 99 is replaced by brand new wood. Again, the old board is hucked out to sea; the new one nailed neatly into place. And so the ship sails on...Yet another year goes by with the ship out to sea, when yet another single board of the ship's original 98 is replaced by brand new wood. Yet again, the old board is hucked out to sea; the new one nailed neatly into place. And so the ship sails on...

So things go for a 100 years, with the Ship of Theseus getting a brand new board to replace one of the older, original ones year after year until 1900. Finally, all that's left to replace is the sail and the mast. And so it happened that in the year 1901, the sail was replaced, and in 1902, the mast.
 

Let's call the ship in 1800: Rod
Let's call the ship in 1902: Todd
 

Question (1): Is Rod identical to Todd?
 

Here are some reasons you might be inclined to answer "yes" to Question (1): (i) we can assume that all of the shipmates aboard the vessel called it "The Ship of Theseus" year after year, (ii) we can assume that no one has gotten off of the ship, or boarded a new one at any point, (iii) we can assume that all of the letters that the shipmates have sent to their familes say things like "yes, I am still aboard this ****ing ship!", etc.
 

But consider the following complication:
 

Sneaky Pete, who is sneaky indeed, followed the ship everywhere it went. Whenever a board was discarded, he collected it, and amassed for himself quite a collection. In 1902, after a hundred and two years of sneaky deeds, all of the discarded parts were collected. Sneaky Pete then assembled a ship the looked much like the one that, in 1800, had first sailed out to sea.

Let's call the ship that Sneaky Pete assembles: Maud
 

Question (2): Is Maud idential to Rod?
 

Here is the main reason whay you might be inclined to answer "yes" to Question (2): (i) Maud and Rod have all of the same parts!
 

But: If Maud is identical to Rod, and Rod is identical to Todd, then (by the transitivity of identity) Maud must be identical to Todd.
 

But this can't be right! For Maud and Todd are two, not one, so they can't be (numerically) identical. [For the distinction between numerical and qualitative identity, go back to the Perry page here.]
 

So: Either our reasons for answering "yes" to Question (1) are bad, or our reasons for answering "yes" to Question (2) are. In other words, either the shipmates do not need to be on the identical ship that they started out on for their claim "We are still on the dang Ship of Theseus" to be true, or being made up of all of the same parts, in exactly the same way, is not enough for identity.
 

Discussion in class.