# Equivalence of Definitions of Consistent Set of Formulas

## Theorem

The following definitions of the concept of Consistent Proof System for Propositional Logic are equivalent:

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

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

Let $\FF$ be a collection of logical formulas.

### Definition 1

Then $\FF$ is consistent for $\mathscr P$ if and only if:

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

That is, some logical formula $\phi$ is not a $\mathscr P$-provable consequence of $\FF$.

### Definition 2

Suppose that in $\mathscr P$, the Rule of Explosion (Variant 3) holds.

Then $\FF$ is consistent for $\mathscr P$ if and only if:

For every logical formula $\phi$, not both of $\phi$ and $\neg \phi$ are $\mathscr P$-provable consequences of $\FF$

## Proof

### Definition 1 implies Definition 2

Suppose that $\FF \not \vdash_{\mathscr P} \phi$.

Aiming for a contradiction, suppose that there exists $\psi$ such that $\FF \vdash_{\mathscr P} \psi$ and $\FF \vdash_{\mathscr P} \neg \psi$.

Then by the Rule of Explosion (Variant 3):

$\psi, \neg \psi \vdash_{\mathscr P} \phi$

and therefore, combining the two above lines:

$\FF \vdash_{\mathscr P} \phi$

$\Box$

### Definition 2 implies Definition 1

According to definition 2, for every logical formula $\phi$ either $\phi$ or $\neg \phi$ is not a $\mathscr P$-provable consequence of $\FF$.

In particular, then, there exists a logical formula $\psi$ such that $\FF \not \vdash_{\mathscr P} \psi$.

$\blacksquare$