# Definition:Axiom/Formal Systems/Axiom Schema

## Definition

Let $\mathcal L$ be a formal language.

Part of defining a proof system $\mathscr P$ for $\mathcal L$ is to specify its axiom schemata.

An axiom schema is a well-formed formula $\phi$ of $\mathcal L$, except for it containing one or more variables which are outside $\mathcal L$ itself.

This formula can then be used to represent an infinite number of individual axioms in one statement.

Namely, each of these variables is allowed to take a specified range of values, most commonly WFFs.

Each WFF $\psi$ that results from $\phi$ by a valid choice of values for all the variables is then an axiom of $\mathscr P$.

## Linguistic Note

The plural of axiom schema is correctly axiom schemata, but it is commonplace to see the word schemas used for schemata.

## Examples

It was proved by Richard Merett Montague in 1957 that ZFC and Peano arithmetic require an axiom schema.