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Does Brook even know that the One Piece is a treasure? Like, he died before Roger even became a famous pirate, and the One Piece (and being the Pirate King) only became a thing after Roger's execution. For all Brook knows, One Piece could be the name of an island full of pirates, like Hachinosu, that Luffy is trying to find so he can become the king of this place, and therefore, the King of the Pirates, or something like that