It seems that the most natural problem in mathematical logic is studying the logics of mathematical theories. If the logics of first-order theories are considered, the situation can be formalized in the following way. Let T be a first-order theory, i.e. a set of closed formulas in a first-order language L. A closed predicate formula is called T-valid if each its closed L-instance is in T. We denote the set of T-valid predicate formulas by L(T) and call it the predicate logic of the theory T.