If temporal parts are bona ﬁde parts, then it is ﬁtting to clarify and extend that notion (and related ones) using the resources of a theory of parts. But it often seems that those engaged in the 3D/4D debate appear to take for granted that, aside from introducing a welcome measure of rigor to the discussion, issues regarding theories of parthood can be allowed to recede into the background. What follows challenges that assumption — I demonstrate that the nature of the fundamental mereological relation1 can decisively inﬂuence the outcome of the debate over persistence. In short, I show that if the fundamental mereological relation is proper parthood-at-atime then four-dimensionalism is false.2 Recognizing this does at least two things for us. First, it supplies a framework in which three-dimensionalists can clarify two things they have tended to say all along, namely that persisting things are ‘wholly present’ throughout their careers and that they do not have temporal parts. Second, it re-focuses the debate on a narrower and perhaps more tractable question: “What is the nature of the fundamental mereological relation?”.