Over time a sailboat decays and need parts replaced for maintenance. Rusted bolts, damaged parts of the hull, peices of the deck, worn fabric in the sail, and so forth are slowly replaced bit by bit over decades until every single part of the ship has been replaced, and nothing is left from the original. After all these replacements, is it the same ship as when it was built? How can it be the same ship if nothing from the original remains? Is it a completely different ship? If so, when did it stop being the old ship and start being the new ship?