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, 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).

I want to inquire into the sources of this conviction, and, by so doing, show that it is groundless.

One of its sources, I think, is the apparently rather common belief that ‘mereological sum’ is, in its primary use, a stand-alone general term like ‘unicorn’ or ‘material object’ – a phrase that picks out a kind of thing, a common noun phrase whose extension comprises objects of a certain special sort. (Or perhaps it is saying toomuch to say that this is a common belief. I might say, more cautiously, that there seems to be a common tendency to presuppose that ‘mereological sum’ is a stand-alone general term, or a common tendency to treat ‘mereological sum’ as a stand-alone general term.)