When developing expert systems, checking knowledge base consistency becomes more and more necessary. As it has been done in a Data Base Management System (DBMS), we propose to define along with the deductive rules, new relations to specify what is permitted and what is forbidden. A definition of knowledge base consistency, integrating these new relations, is presented, based on classical logic. We expose a method to prove this property, constructing new structures which model the knowledge base. A theorem is demonstrated which shows that proving the knowledge base consistency is equivalent to proving the consistency of our structures, which is easier to do. Finally, a method which allows an incremental construction of the knowledge base is presented.