Ship of Theseus

An ancient Greek legend refers to a certain ship ridden home by the Greek hero Theseus. Over the years, the ship's planks decayed, but the people of Athens, in honor of their hero, preserved it by replacing the planks with new ones. This went on, and eventually came to the point where the ship was then composed of completely different parts from when it was first built.

Was this still Theseus's ship?

And if it wasn't. How about if it had one original plank? Or two?