[For the entire collection see Zbl 0578.00001.]
In the paper we attempt to define a logic to reason about entities which are defined up to indiscernibility relations. Indiscernibility relations are assumed to be determined by properties of entities. The logic is a multimodal logic such that accessibility relations corresponding to modal operations are determined by sets of parameters. The treatment of indiscernibility presented in the present paper is inspired by the rough set approach to knowledge representation. The focus in this paper is on definability of entities and sets of entities up to indiscernibility.