Questionnaire-2024/natural_deduction_predicate_logic/0008_sol.tex

This sequent is not provable.

Model $\mathcal{M}$:\\
\begin{minipage}{0.2\textwidth}
  \begin{align*}
  \mathcal{A} =& \; \{ a,b \}\\
  P^\mathcal{M} =& \; \{ a \}\\
  Q^\mathcal{M} =& \; \{ a \}\\
\end{align*}
\end{minipage}\\

$\mathcal{M} \models \forall x \; (P(x)\imp Q(x)), \quad \exists x \; P(x)$\\
$\mathcal{M} \nmodels \forall x Q(x)$