# Definition:Classes of WFFs/Plain Sentence

## Definition

Let $\LL_1$ be the language of predicate logic.

A WFF is said to be a plain sentence if and only if it is both plain and a sentence.

That is, if and only if it contains free variables nor parameters.

Thus, plain sentences are those WFFs which are in $\map {SENT} {\PP, \FF, \O}$.