Theory of Set of Formulas is Theory
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Theorem
Let $\LL$ be a logical language.
Let $\mathscr M$ be a formal semantics for $\LL$.
Let $\FF$ be a set of $\LL$-formulas.
Let $\map T \FF$ be the $\LL$-theory of $\FF$.
Then $\map T \FF$ is a theory.
Proof
Let $\phi$ be a $\LL$-formula.
Suppose that $\map T \FF \models_{\mathscr M} \phi$.
By definition of $\map T \FF$:
- $\FF \models_{\mathscr M} \psi$
for every $\psi \in \map T \FF$.
Hence by definition of semantic consequence:
- $\FF \models_{\mathscr M} \map T \FF$
By Semantic Consequence is Transitive, it follows that:
- $\FF \models_{\mathscr M} \phi$
Finally, by definition of $\map T \FF$:
- $\phi \in \map T \FF$.
Since $\phi$ was arbitrary, the result follows.
$\blacksquare$
Sources
- 2012: M. Ben-Ari: Mathematical Logic for Computer Science (3rd ed.) ... (previous) ... (next): $\S 2.10$: Exercise $2.17$