With the pressing environmental problems currently facing the urbanization, urban system in view of biophysics concerning energy and materials flows and related hierarchical structure is necessary to be explored. Emergy as a type of embodied energy is therefore be employed as a powerful tool to solve urban system modeling issues. This paper aims to delineate the recent progress of urban studies based on emergy, and back-cast the threads in terms of Aristotelian causality. The applications of

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... rgy to urban system can be thereby divided into four categories, which may provide a new lens for the urban system planning and management.