 Abelard’s theory of substantial integral wholes is not a pure mereology in the modern sense, since he holds that there are privileged divisions: just as a genus is properly divided into not just any species but its proximate species, so too the division of a whole must be into its principal parts. Intuitively, some wholes have a natural division that takes precedence over others; a sentence, for example, is divided into words, syllables, and letters, in precisely that order. According to Abelard, the principal parts of a whole are those whose conjunction immediately results in the complete whole. His intent seems to be that the nature of the composition (if any) that defines the integral whole also spells out its principal parts. A house consists of floor, walls, and roof put together in the right way. It is an open question whether each principal part (such as the wall) requires the existence of all of its subparts (every brick). The principal parts of a collection, for example, are just each of the members of the collection, whatever may be the case with any given member’s subparts; the principal parts of an aggregation are the members located in proximity to one another.