Next, we try to work out f. We confront the same problem as that in the case of cosmological constant: there is no proper defi-nition of surface gravity in modified gravity. Similar to the case of the cosmological constant, we also have two perspectives about modified gravity theory. The first perspective is to treat all the terms generated by the action other than Einstein tensor as stress energy of “matter fields”. Actually, this perspective has been ex-tensively explored in cosmology. Usually, all the terms other than Einstein tensor is treated as effective dark energy to investigate the cosmic acceleration [19], where this perspective is called Einstein interpretation. The second perspective is to treat all the geometric sector as gravity. That is the natural modified gravity perspective.