  The task of characterizing reduction is more involved when the reduction is heterogeneous—that is, when the reduced theory contains terms or concepts that do not appear in the reducing theory. Nagel takes, as a paradigm example of heterogeneous reduction, the (apparent) reduction of thermodynamics, or at least some parts of thermodynamics, to statistical  mechanics.[1]  For instance, thermodynamics contains the concept of temperature (among others) that is lacking in the reducing theory of statistical mechanics.