No Boolean Interpretation Models a WFF and its Negation
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Theorem
Let $v$ be a boolean interpretation.
Let $\mathbf A$ be a WFF of propositional logic.
Then $v$ can not model both $\mathbf A$ and $\neg \mathbf A$.
Proof
Suppose that $v$ models $\mathbf A$:
- $v \models \mathbf A$
Then $v \left({\mathbf A}\right) = T$ by definition of models.
By definition of boolean interpretation, $v \left({\neg \mathbf A}\right) = F$.
In particular, $v (\neg \mathbf A) \ne T$, so that:
- $v \not\models \neg \mathbf A$
Hence the result.
$\blacksquare$