"If all parts are replaced, is it still the same ship?"
Plutarch tells of the ship of Theseus, preserved by Athenians. Over time, rotting planks were replaced one by one until every single piece of wood was new. Is it still the same ship? If not, when did it cease to be the same?
This paradox questions the nature of identity. If identity is based on substance (material), it is a different ship. If identity is based on continuity of form (structure/history), it is the same ship. Consider that almost all cells in your body are replaced every 7-10 years. Are you the same person you were a decade ago?
Thomas Hobbes added a twist: If someone gathered all the old, discarded planks and built a ship, which one is the 'real' Ship of Theseus? The renovated one or the reconstructed one? This highlights the ambiguity of the word 'same'.