 In this sense, Nagel reduction is sometimes (cf. Schaffner 1967: 137, 1993: 423) interpreted as a kind of direct reduction, in contrast with indirect reductions—those that hold in virtue of the fact that the reducing theory explains the occurrence of the phenomena of the reduced theory (rather than the reduced theory itself)—a view associated with Kemeny and Oppenheim (1956) and Friedman (1982), and captured as one variant of reduction by Schaffner’s model (Schaffner 1993).