Propositional proof systems can be interpreted as nondeterministic algorithms for recognizing tautologies. In propositional logic, they are tautologies. Equivalently, all tautologies of sentential logic can be taken as axioms. Thus non-logical axioms, unlike logical axioms, are not "tautologies". Surprisingly, the logical truths (or tautologies) of LP are precisely those of classical propositional logic. These formulas play a role similar to tautologies in propositional logic. In natural languages, some apparent tautologies may have non-tautological meanings in practice. In the deduction apparatus of necessity logic the logical axioms are the usual classical tautologies. In this context, logical tautologies, and purely mathematical propositions may be regarded as "nonsense". Then sequents signify conditional theorems in a first-order language rather than conditional tautologies. A formal system is considered semantically complete when all of its theorems are also tautologies. Logical truths (including tautologies) are truths which are considered to be necessarily true. The difference between the two logics lies in how tautologies are defined. Not all logical truths are tautologies of such a kind. Utterances that are not informative on the surface include tautologies.