 Identity is, very plausibly, an equivalence relation (see  Section 2.2).  One consequence of it being an equivalence relation is that it is subject to transitivity: the principle that if aRb and bRc, then aRc. Saying that, in the case of Theseus' ship, Replacement and Reassembly are both identical with the original ship putatively conflicts with the transitivity of identity since Replacement seems to be later clearly distinct from Reassembly.