The main aim of this book is to present recent ideas in logic centered
around the notion of a consequence operation. We wish to show these
ideas in a factually and materially connected way, i.e., in the form of
a consistent theory derived from several simple assumptions and
definitions. These ideas have arisen in many research centers. The
thorough study of their history can certainly be an exciting task for
the historian of logic; in the book this aspect of the theory is being
played down. The book belongs to abstract algebraic logic, the area of
research that explores to a large extent interconnections between
algebra and logic. The results presented here concern logics defined in
zero-order languages (Le., quantifier-free sentential languages without
predicate symbols). The reach of the theory expounded in the book is, in
fact, much wider. The theory is also valid for logics defined in
languages of higer orders. The problem of transferring the theory to the
level of first-order languages has been satisfactorily solved and new
ideas within this area have been put forward in the work of Blok and
Pigozzi [1989].