 The quantification is restricted to bridge-laws of the appropriate kind. A variant of this definition interprets the relata as sets of laws of two theories. Nagel’s homogeneous reductions are trivially covered (bridge-laws are irrelevant for the derivation; however, they do exist and state trivial connections not connecting distinct predicates, but the very same predicate in the reduced and the reducing theory). Variants of this definition include Schaffner’s model that, in addition, mentions possible replacements and catches aspects of the epistemological dimension of reductions, concerning the way scientists actually arrive at reductions (Schaffner 1993). These additional requirements might be regarded as imposing further constraints on the reductive link; however, especially in Schaffner’s model as well as in New Wave models of reduction, these further constraints are always disjunctive and seem to play the role of characterizations that accompany the definition. They do not form necessary conditions for reduction.