of various objects of the world, their mutual relationship, laws governing their behavior and evolution. For example, consider a system $S$ of medical knowledge about the “world” of a hospital. $S$ contains description of diseases (their causes, development, consequences, examination, symptoms, treatment, prevention), information about various medicament (their composition, therapeutic activity, dosage, directions for use, interactions, side effects), description of the hospital (its structure, management, location), personal data of the hospital patients (their medical history, test results), general rules of medicine, etc. An important decision that has to be made by a physician for his or her patient is determining the right diagnosis and the best treatment. The physician may wish to consult the vast amount of knowledge collected in the system. Will the system help the physician to make a right decision? This depends to a large extent on the way the knowledge is used for deriving conclusions.

Let us define important features of $S$ and their correspondence to the world it describes.

> $S$ is presented in a first order language. $S$ is consistent having a set $MOD(S)$ of models. Each model of $S$ is a set of ground atomic formulas expressed in terms of values assigned to 