In mathematical logic, a set ${\displaystyle {\mathcal {T))}$ of logical formulae is deductively closed if it contains every formula ${\displaystyle \varphi }$ that can be logically deduced from ${\displaystyle {\mathcal {T))}$, formally: if ${\displaystyle {\mathcal {T))\vdash \varphi }$ always implies ${\displaystyle \varphi \in {\mathcal {T))}$. If ${\displaystyle T}$ is a set of formulae, the deductive closure of ${\displaystyle T}$ is its smallest superset that is deductively closed.

The deductive closure of a theory ${\displaystyle {\mathcal {T))}$ is often denoted ${\displaystyle \operatorname {Ded} ({\mathcal {T)))}$ or ${\displaystyle \operatorname {Th} ({\mathcal {T)))}$.[citation needed] This is a special case of the more general mathematical concept of closure — in particular, the deductive closure of ${\displaystyle {\mathcal {T))}$ is exactly the closure of ${\displaystyle {\mathcal {T))}$ with respect to the operation of logical consequence (${\displaystyle \vdash }$).

## Examples

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.

## Epistemic closure

 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.