# Definition:Classes of WFFs/Plain WFF

## Definition

A **plain WFF** of predicate logic is a WFF with no parameters.

Thus $WFF \left({\mathcal P, \mathcal F, \varnothing}\right)$ is the set of all **plain WFFs** with relation symbols from $\mathcal P$ and function symbols from $\mathcal F$.

It is immediate that a **plain WFF** is a WFF with parameters from $\mathcal K$ for *all* choices of $\mathcal K$.

## Sources

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