 With these caveats in place, we can begin to explore the basic mereological categories that one finds in medieval treatments of mereology. As was noted, all manner of items can be wholes, but generally speaking medieval philosophers believe that this motley group can be organized under three broad categories: integral wholes, universal wholes, and potential wholes. It will soon become apparent that the category of integral wholes is quite broad. Indeed, it is so inclusive that some philosophers will feel pressure to introduce a fourth basic category, substantial wholes. But others will suggest that an integral whole is divisible into two distinct kinds of parts, quantitative parts and substantial parts. For this reason, I will consider the issue of substantial wholes and substantial parts in  Section 3.1.  