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4. Formal Logic / B. Propositional Logic PL / 4. Soundness of PL
[any formula which has a proof is a valid formula]
2 ideas
9536
If any of the nine rules of propositional logic are applied to tautologies, the result is a tautology
[Lemmon]
9543
The rules preserve validity from the axioms, so no thesis negates any other thesis
[Hughes/Cresswell]