Eclectic Anecdotes

Plutarch wrote in his The Life of Theseus:

The ship on which Theseus sailed with the youths and returned in safety, the thirty-oared galley, was preserved by the Athenians down to the time of Demetrius Phalereus. They took away the old timbers from time to time, and put new and sound ones in their places, so that the vessel became a standing illustration for the philosophers in the mooted question of growth, some declaring that it remained the same, others that it was not the same vessel.[1]

Thomas Hobbes would elaborate on this philosophical exercise on identity in his book De Corpore:

According to the second Opinion, two Bodies existing both at once, would be one and the same Numerical Body; for if (for example) that Ship of Theseus (concerning the Difference whereof, made by continual reparation, in taking out the old Planks, and putting in new, the Sophisters of Athens were wont to dispute) were, after all the Planks were changed, the same Numerical Ship it was at the beginning; and if some Man had kept the Old Planks as they were taken out, and by putting them afterwards together in the same order, had again made a Ship of them, this without doubt had also been the same Numerical Ship with that which was at the beginning; and so there would have been two Ships Numerically the same, which is absurd.[2]

This paradox is solved by understanding that a ship is an artifact created by humans. Therefore, its definition is supplied by humans and it does not have a concrete universal. The identity, then, is by whatever conditions are applied to each particular instance. For example, if a certain ship is defined by having x amount of original planks, then if it loses any, it cannot be said to be the same ship.

But this paradox is more interesting if the artifact aspect is ignored and used as an analogy to understand human identity. Billions of human cells die and are regenerated daily. Although the material body is constantly in flux, the essence of the particular human does not change. The form of the human maintains the secondary substance (the particular), even while these cells are replaced. Replacing parts of the person does not negate the unique individual. The form is retained, even though the matter changes. If, theoretically, all of these cells which are to be replaced could be united to make a new person, essentially a clone, so there would now be two similar persons existing simultaneously, it would not follow that they are the same person, since a clone is a duplicate of the original.

Ships are also constantly changing, the matter slowly being battered and eroded by the elements. As the planks are replaced, the form of the particular ship retains its individuality, just as the particular human does as cells are replaced. Likewise, if a new ship is assembled from the old planks, it becomes a duplicate of the original ship which now has the new planks, even though it looks identical to the original. The changing planks are only accidental forms of the ship; the substantial form is the ship itself. Again, the form of the ship itself does not change, but the material cause (planks) changes.

[1] http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Plutarch/Lives/Theseus*.html

[2] Hobbes, Thomas. Delphi Collected Works of Thomas Hobbes (Illustrated) (Delphi Series Ten Book 9) (p. 2299). Delphi Classics. Kindle Edition.