 Many four-dimensionalists subscribe to a principle that a number of writers call unrestricted mereological composition. According to that principle any set of, in this case four-dimensional, objects constitute a further four dimensional object. Armed with the unrestricted mereological composition principle a four-dimensionalist can say that what there is, in the case of Theseus' ship, is a Y branching object dividable in an indefinite number of ways into four dimensional proper parts an indefinite number of which are ships. For example the ship consisting of the stem together with the replacement branch has as two of its proper parts a ship consisting of just the stem, and one consisting of just the replacement branch.