  It is proved that every prevariety of algebras is categorically equivalent to
a "prevariety of logic", i.e., to the equivalent algebraic semantics of some
sentential deductive system. This allows us to show that no nontrivial equation
in the language "meet, join, and relational product" holds in the congruence
lattices of all members of every variety of logic, and that being a
(pre)variety of logic is not a categorical property.
