Describes deductive systems whose variables range over propositions (or sentences) rather than individuals, properties, etc, i.e., logics over a propositional language. Typically, the primitive atomic formulae in propositional logics are sentence letters or parameters, e.g., $p$, $q$, $r$, etc. The expressivity of a propositional logic is limited, but may be enhanced by extending the basic logic with terms representing individuals and properties, quantifiers, and so forth.