 Secondly, this way of conceiving of the primary relata of reduction suggests that the reduction-relation is a binary relation. Recall, however, the observation that on Nagel’s account, in heterogenic cases of reduction, no theory reduces to another theory simpliciter; it does so only with respect to certain bridge-laws and/or boundary conditions. Depending on how one counts the arity of a relation, i.e., the number of items it relates, this might suggest that even on the classical interpretation, reduction is not a binary relation. More recently, alternative explications have been suggested according to which reduction is of an arity higher than 2. Thus, the intuitive notion of primary relata should be tied to the designata of the expressions flanking the reduction predicate in true reduction statements.