In many computer applications we face the following situation: we are given set of objects which are characterized by means of some features like: temperature, height, weight etc., and we want to dene in terms of those features some concepts. For example we have a data le concerning patients suering from a certain disease. State of each patient is characterized by some symptoms like, for example: blood pressure, temperature etc. The question arises whether wa are able to dene this disease (...) in terms of avail- able symptoms. Notice that we identify here concept of a specic disease with a set of patients suering from this disease. Hence the denition of a disease depends upon specic example of a disease under consideration. This approach refers to inductive reasoning, when a concept is built upon the basis of a nite number of examples of this concept. More detailed discussion of problems considered here one { can nd in Pawlak , Pawlak and Or lowska and Pawlak. (shrink)