Why Combine Logics?

Combining logics has become a rapidly expanding enterprise that is inspired mainly by concerns about modu larity and the wish to join together tailored made logical tools into more powerful but still manageable ones A natural question is whether it o ers anything new over and above existing standard languages By analysing a number of applications where combined logics arise we argue that combined logics are a potentially valuable tool in applied logic and that endorsements of standard languages often miss the point Using the history of quanti ed modal logic as our main example we also show that the use of combined structures and logics is a recurring theme in the analysis of existing logical systems AMS Subject Classi cation A B CR Subject Classi cation F F I