The series of questions that was set out in the preceding paragraph was introduced by observing that the problem of universals includes both questions about the existence and nature of universals and questions about how universals are related to the particulars that fall under them. Many of the theories that were alluded to in that series of questions could be described as theories of the “ontological structure” of non-universals. We can contrast ontological structure with mereological structure. A philosophical question concerns the mereological structure of an object if it is a question about the relation between that object and those of its constituents that belong to the same ontological category as the object. For example, the philosopher who asks whether the Taj Mahal has a certain block of marble among its constituents essentially or only accidentally is asking a question about the mereological structure of the Taj, since the block and the building belong to the same ontological category. But the philosopher who asks whether the Taj has “whiteness” as a constituent and the philosopher who supposes that the Taj does have this property-constituent and asks, “What is the nature of this relation ‘constituent of’ that ‘whiteness’ bears to the Taj?” are asking questions about its ontological structure.