Definition:Inconsistent (Logic)/Definition 2

Definition

Let $\LL$ be a logical language.

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

A set $\FF$ of logical formulas is inconsistent for $\mathscr P$ if and only if:

There exists a logical formula $\phi$ such that both

$\FF \vdash_{\mathscr P} \paren {\phi \land \neg \phi}$

Also known as

Inconsistent sets of logical formulas are often called contradictory.

Likewise, a logical formula which is inconsistent by itself is often called a contradiction.

Since these terms are also often used to describe unsatisfiability in the context of a formal semantics, they are discouraged as synonyms of inconsistent on $\mathsf{Pr} \infty \mathsf{fWiki}$.

Also see

• Equivalence of Definitions of Inconsistent (Logic)

• Results about inconsistent in the context of logic can be found here.

Sources

• 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): contradiction
• 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): contradiction