Suppose there is a ship called 'The Ship of Theseus.' And let us suppose that this ship is made up of 100 parts--98 planks, and a sail and a mast, say. In the year 1900, the ship goes out to sea. A  
    year goes by, when someone decides one of the planks should be replaced. So a single plank of the ship's original 98 is replaced by a brand new one. The old board is chucked out to sea; the new     one nailed neatly into place. And so the ship sails on.

    Another year goes by. But someone again decides to replace one of the original planks with a new one. So the old board is chucked out to sea; a new one is 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 chucked out to sea; the new one
    nailed neatly into place. And so the ship sails on. And so 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 finally even the mast and the sail are eventually
    replaced. In the year 2000, there are no original parts left.  

   
    Let's call the ship in 1900: Rod
    Let's call the ship in 2000: 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 families 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 of Theseus everywhere it went. Whenever a board was discarded, he collected it, and squirreled it away in some cave. In 2000, after a        
    hundred 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 1900, had first sailed out to sea.

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

Question 2: Is Maud identical to Rod?  

    Here is the main reason why 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 to the Perry page here.) 

    So: Either our reason for answering "yes" to Question 1 is 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.