 In recent years, further worries have been raised concerning mereological theories with substantive composition principles—especially concerning the full strength of GEM. Among other things, it has been argued that the principle of unrestricted composition does not sit well with certain fundamental intuitions about persistence through time (van Inwagen 1990, 75ff), that it is incompatible with certain plausible theories of space (Forrest 1996b), or that it leads to paradoxes similar to the ones afflicting naïve set theory (Bigelow 1996). A detailed examination of such arguments is beyond the scope of this entry. For some discussion of the first issue, however, see Rea (1998), McGrath (1998, 2001), Hudson (2001: 93ff) and Eklund (2002: §7). On the second, see Oppy (1997) and Mormann (1999). Hudson (2001: 95ff) also contains some discussion of the last point.