 Thus, Nagel-reduction is a relation that holds not just between theories but also between their ontologies. Sometimes, however, the Nagel model has been characterized as an epistemological model of reduction, because it is a model of theory-reduction (Sarkar 1992; Hoyningen-Huene 1989; Silberstein 2002), because it is a model of explanatory reduction (Sarkar 1992), or because bridge-laws are to be interpreted epistemologically (Fazekas 2009, following Klein 2009).