On the specification and knowledge of sets

The author examines and explicates the intuitive concept of the knowledge a set. He points to the problems of set specification by means of empirical conditions or by means of expressions, of which the referent is world-depen dent. Attention is paid also to the Russellian concept of acquaintance with a set. The last part of the paper shows various explications of the concep t of the knowledge a set, corresponding to different levels of the effectiv eness of conditions (in Leene-Mostowski Hierarchy), by means of which the s ets are specified.