Imagine a ship, whose sole function is to make a yearly voyage to a neighbouring country in order to honour a heroic deed from the past. The ship in question is composed of wooden planks, and her shape might be described as very distinctive. After a few years of making her yearly voyage, the ship’s planks begin to weather. The crew decides that henceforward, before the ship sets sail each year, they will replace the weathered planks of the ship with new ones. Eventually, all of the planks of the original ship are replaced. Now someone (say her name is Merry) collects the planks that are disposed of from the original ship each year, until some years later, Merry has collected all of the planks from the original ship. Furthermore, Merry decides to put the planks she has collected together in her back yard, giving those planks the same distinctive configuration they had when they composed the original ship at the time of her first voyages. Given the information in this story, someone might well wonder which ship is numerically identical to the original ship. Is it the continuous ship, which continues to make the yearly voyage to the neighbouring country and whose spatio-temporal history is continuous with that of the original ship, or is it the reconstructed ship, which is composed of the same set of planks as the original ship? Indeed, they cannot both be numerically identical to the original ship, since the continuous ship is out to sea, and the reconstructed ship resides in Merry’s (very dry) back yard!
– Christopher M. Brown, Aquinas and the Ship of Theseus, 2005