 As we have seen, Burley thinks that what falls under any categorial field are simple accidental forms; therefore, the things of the ad aliquid category are relations (relationes) and not relatives (relativa or ad aliquid), which are merely aggregates formed by a substance and a relation. Accordingly, the relationship between relation and relatives is like that between quantity and what is quantified, or quality and what is qualified. The relation is the cause of the nature of the aggregate (that is, the relatives), of which it is a constituent. Unlike modern logicians, Burley denies that a relation is a two-place predicate, and views it instead as a monadic function, arguing that like the other accidental forms, relation inheres in a single substrate and makes reference to another thing without inhering in it. This thesis is based on the following principle, which Burley states in his commentary on the Book of Six Principles (LsP, ch. de habitu, fol. 63ra): there must always be equivalence and correspondence between the accidental form and its substrate of inherence, so that no accidental form can completely inhere at the same time in two or more different substrates—not even numbers, the different parts of which inhere in their own distinct substrates. Unlike other accidental forms, however, relations do not directly inhere in their substrates, but are present in them only by means of another accidental form that Burley calls the foundation of the relation (fundamentum relationis—EP, ch. de relatione, fol. 34va). Among the nine categories of accidents, only quantity, quality, action, and affection can be the foundations of relations.