We provide a natural extension of the intuitionistic natural deduction system NJ with at most two consequences, which we call a binary conclusion natural deduction system. The classical natural deduction system NK is defined as NJ together with the law of the excluded middle or the double negation principle. However, in contrast to NK, the new BCN system has no such classical rule, but has two kinds of rules managing two consequences. One is to eliminate one consequence and the other is to intro…
Read moreWe provide a natural extension of the intuitionistic natural deduction system NJ with at most two consequences, which we call a binary conclusion natural deduction system. The classical natural deduction system NK is defined as NJ together with the law of the excluded middle or the double negation principle. However, in contrast to NK, the new BCN system has no such classical rule, but has two kinds of rules managing two consequences. One is to eliminate one consequence and the other is to introduce another consequence. Except on this point, BCN proofs are derived in exactly the same way as those in NJ. It is shown that with respect to the propositional fragment of NK, the BCN system is sound and sufficiently complete to prove classical propositional theorems. Moreover, as a corollary we obtain a general form of Glivenko's theorem and disjunction property for one of the two consequences. We also show the normalization theorem for BCN. Finally, we study some extensions of BCN to classical predicate logic
