Representations of the lower and upper approximations of a set in the context of an approximation space as modal operators in the first order language of modal logics, are quite natural and widely familiar now to the rough-set community. According to the perception of an observer, objects of a universe (of discourse) are clustered. These are the basic information granules (or quanta). With respect to the information available, objects belonging to the same cluster are indistinguishable. It may not always be the case that the clusters are mutually disjoint.