Definition:Consistent (Logic)/Proof System/Propositional Logic/Definition 1

Definition
Let $\LL_0$ be the language of propositional logic.

Let $\mathscr P$ be a proof system for $\LL_0$.

Then $\mathscr P$ is consistent :


 * There exists a logical formula $\phi$ such that $\not \vdash_{\mathscr P} \phi$

That is, some logical formula $\phi$ is not a theorem of $\mathscr P$.