\begin{definition}[Definition:Classes of WFFs/Plain Sentence]
Let $\LL_1$ 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, {{iff}} it contains free variables nor parameters.
Thus, '''plain sentences''' are those WFFs which are in $\map {SENT} {\PP, \FF, \O}$.
\end{definition}