First-Order Logic

Entry conditions

Use first-order logic when you need to talk about objects and their relations, not just whole statements.

Definitions

• A first-order language includes variables, function symbols, relation symbols, and quantifiers.
• A structure interprets these symbols on a domain of objects.

Vocabulary (plain language)

• Domain: the set of objects you are talking about.
• Interpretation: assignment of meaning to symbols in the language.
• Quantifier: “for all” ($\forall$) or “there exists” ($\exists$).

Symbols used

• $\forall x$: for all $x$
• $\exists x$: there exists $x$

Intuition

First-order logic lets you express properties of objects and relations between objects, like “every trace has a cover” or “there exists a stabilizer.”

Worked example

Let the domain be natural numbers. The statement $\forall x \exists y (y = S(x))$ says every number has a successor.

How to recognize the structure

• You can define a domain of objects.
• You can interpret relation and function symbols.

Common mistakes

• Using first-order logic when higher-order quantification is required.