 Here is what a four dimensionalist such as Lewis will say about Theseus' ship. In that case there are two initially indiscernible ships. The first is a four dimensionally extended ship located where the original ship is initially located that ends up coinciding only with the reassembly ship. The second, also initially located where the original ship is initially ends up coinciding with the replacement ship. We thus have a Y-branching four dimensionally extended object with one ship constituted by the stem together with one branch, and the other constituted by the stem together with the remaining branch.