Many philosophers think not. Many philosophers, in fact, seem to suppose that anyone who raises the question whether mereological sums can change their parts displays thereby a failure to grasp an essential feature of the concept “mereological sum.” It is hard to point to an indisputable example of this in print,[i] but it is a thesis I hear put forward very frequently in conversation (sometimes it is put forward in the form of an incredulous stare after I have said something (...) that implies that mereological sums can change their parts). (shrink)