In propositional logic, the set of all true propositions is deductively closed. This is to say that only true statements are derivable from other true statements.
Main article: Epistemic closure
In epistemology, many philosophers have and continue to debate whether particular subsets of propositions—especially ones ascribing knowledge or justification of a belief to a subject—are closed under deduction.
First-order theory at PlanetMath. https://planetmath.org/firstordertheory ↩