 However, some have argued that the distinction itself may not be viable (van Riel 2011). First, it is not clear what it is to explain a theory (except for explaining it to, say, a student). Second, the examples Nagel discusses suggest that he believed his model to be indirect—an “explanation” of a theory in terms of derivation from a reducing theory amounts to using the reducing theory to explain the phenomena dealt with by the reduced theory.