Our everyday thinking consists of two steps: "forward" extending our beliefs, "backward" reducing them. The "forward" step is formalized by deductive logic, but existing logics formalising "rejected sentences" reasoning are unvalid for the "backward" reasoning. We need two logics: one for the set of accepted sentences, another for the set of rejected sentences. They work on the same class of sets, so the second component of the pair must be a reasoning decreasing sets of accepted sets.