# Definition:Classes of WFFs/Plain Sentence

< Definition:Classes of WFFs(Redirected from Definition:Plain Sentence)

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## Definition

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

A WFF is said to be a **plain sentence** iff it is both plain and a sentence.

That is, if it contains free variables nor parameters.

Thus, **plain sentences** are those WFFs which are in $SENT \left({\mathcal P, \mathcal F, \varnothing}\right)$.

## Sources

- 1996: H. Jerome Keisler and Joel Robbin:
*Mathematical Logic and Computability*: $\S 2.3$