# Definition:Structure for Predicate Logic

*This page is about structures in the context of predicate logic. For other uses, see Definition:Structure.*

## Contents

## Definition

Let $\mathcal L_1$ be the language of predicate logic.

A **structure $\mathcal A$ for $\mathcal L_1$** comprises:

- $(1):$ A non-empty set $A$;
- $(2):$ For each function symbol $f$ of arity $n$, a mapping $f_{\mathcal A}: A^n \to A$;
- $(3):$ For each predicate symbol $p$ of arity $n$, a mapping $p_{\mathcal A}: A^n \to \Bbb B$

where $\Bbb B$ denotes the set of truth values.

$A$ is called the **underlying set** of $\mathcal A$.

$f_{\mathcal A}$ and $p_{\mathcal A}$ are called the **interpretations** of $f$ and $p$ in $\mathcal A$, respectively.

We remark that function symbols of arity $0$ are interpreted as constants in $A$.

Also, the predicate symbols may be interpreted as relations via their characteristic functions.

### Formal Semantics

#### Formal Semantics for Sentences

The structures for $\mathcal L_1$ can be interpreted as a formal semantics for $\mathcal L_1$, which we denote by $\mathrm{PL}$.

For the purpose of this formal semantics, we consider only sentences instead of all WFFs.

The structures of $\mathrm{PL}$ are said structures for $\mathcal L_1$.

A sentence $\mathbf A$ is declared ($\mathrm{PL}$-)valid in a structure $\mathcal A$ if and only if:

- $\operatorname{val}_{\mathcal A} \left({\mathbf A}\right) = T$

where $\operatorname{val}_{\mathcal A} \left({\mathbf A}\right)$ is the value of $\mathbf A$ in $\mathcal A$.

Symbolically, this can be expressed as:

- $\mathcal A \models_{\mathrm{PL}} \mathbf A$

#### Formal Semantics for WFFs

The structures for $\mathcal L_1$ can be interpreted as a formal semantics for $\mathcal L_1$, which we denote by $\mathrm{PL_A}$.

The structures of $\mathrm{PL_A}$ are pairs $\left({\mathcal A, \sigma}\right)$, where:

- $\mathcal A$ is a structure for $\mathcal L_1$
- $\sigma$ is an assignment for $\mathcal A$

A WFF $\mathbf A$ is declared ($\mathrm{PL_A}$-)valid in a structure $\mathcal A$ if and only if:

- $\sigma$ is an assignment for $\mathbf A$
- $\mathop{ \operatorname{val}_{\mathcal A} \left({\mathbf A}\right) } \left[{\sigma}\right] = T$

where $\mathop{ \operatorname{val}_{\mathcal A} \left({\mathbf A}\right) } \left[{\sigma}\right]$ is the value of $\mathbf A$ under $\sigma$.

Symbolically, this can be expressed as one of the following:

- $\mathcal A, \sigma \models_{\mathrm{PL_A}} \mathbf A$

- $\mathcal A \models_{\mathrm{PL_A}} \mathbf A \left[{\sigma}\right]$

## Also known as

A **structure for $\mathcal L_1$** is also often called a **structure for predicate logic** or **first-order structure**.

The latter formulation is particularly used when the precise vocabulary used for $\mathcal L_1$ is not important.

## Also see

## Sources

- 2009: Kenneth Kunen:
*The Foundations of Mathematics*... (previous) ... (next): $\mathrm{II}.7$ First-Order Logic Semantics: Definition $\mathrm{II}.7.1$