In natural deduction classical logic is commonly formulated by adding a rule such as Double Negation Elimination (DNE) or Classical Reductio ad Absurdum (CRA) to a set of introduction and elimination rules sufficient for intuitionist first-order logic with conjunction, disjunction, implication, negation and the universal and existential quantifiers all taken as primitive. The natural deduction formulation of intuitionist logic, coming from Gentzen, has nice properties:— (i) the separation property: an intuitionistically valid inference is derivable using only the introduction and elimination rules governing the connectives and/or quantifiers that occur in the premises (if any) and conclusion; (ii) the (strict) subformula property: more narrowly, there is a derivation of any intuitionistically valid inference that employs only subformulas of the formulas occurring in premises (if any) and conclusion. (Every formula is, of course, a subformula of itself.).
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