1 Felucca owner: My great great grandfather sailed this very felucca.
1 Monty: It looks almost new.
2 Felucca owner: Oh, the timbers, mast, sail, and ropes have all been replaced many times.
2 Monty: But... it’s not the same boat then!
3 Prof. Jones: It’s like the Ship of Theseus! This is exactly the same argument people have been having since antiquity.
4 Monty: No it’s not.
4 Prof. Jones: Yes it is! It’s the same argument, only the participants have been replaced!

The Ship of Theseus is a philosophical question that has been discussed since at least the time of Heraclitus and Plato, around 500-400 BC. The formulation in terms of a ship belonging to the mythological figure of Theseus comes from traditional Greek mythology, and was recorded by Plutarch:

The ship wherein Theseus and the youth of Athens returned from Crete had thirty oars, and was preserved by the Athenians down even to the time of Demetrius Phalereus, for they took away the old planks as they decayed, putting in new and stronger timber in their places, insomuch that this ship became a standing example among the philosophers, for the logical question of things that grow; one side holding that the ship remained the same, and the other contending that it was not the same.

Most people would probably agree that if you own a ship (or a felucca) and, say, one nail becomes rusty so you pull it out and put in a new nail, then it's still the same ship (or felucca). The Ship of Theseus asks what happens if over many years you slowly replace one part after another as they wear out (very likely never replacing more than a fraction of a percent at a time) until you reach a point where none of the original parts remain. At this point, is it still the same ship?

Now, if you actually owned a boat and you had it registered, and you replaced worn parts one at a time over the years... I have no doubt that both the legal authorities and you would still consider it the same registered boat at all times. And if you reach the point where all of the parts have been replaced so that none of the original boat remained, it would still be registered under the name of the original boat. But looking back and thinking about it, would you as the owner consider it to be the same boat?

Now imagine, as Thomas Hobbes did in the 17th century, that you kept all of the removed parts that you replaced over the years. And once the registered boat consisted of all new parts, you took all the original parts that had been removed and reassembled them to make another boat (albeit a bit worn out). Now you have two boats. Which one is the original boat? The newer one that has been continuously legally registered in the same name, or the unregistered worn out one made from the original parts?

There's no answer, as people disagree.

Lest this be considered a thought experiment with no real world application, consider the famous submersible Alvin, operated by the Woods Hole Oceanographic Institution and responsible for many pioneering oceanographic discoveries. Since it was built in 1964, it has been repaired, refitted, and overhauled many times, and in fact every component of the original 1964 Alvin has been replaced. Alvin still operates now, and has always been registered as the same vessel, without interruption from 1964 to now. Is it the same vessel?

Again, people may disagree.

This argument has been raging for at least 2400 years. The same argument, just with different people.