Onset and Today’s Perspectives of Multilevel Syllogistic

We report on the initial phases of a systematic study (undertaken over forty years ago) on decidable fragments of Set Theory, to which Alfredo Ferro contributed and which later branched out in many directions. The impact that research has had so far and will continue to have, mainly in the areas of proof-checking, program-correctness verification, declarative programming—and, more recently, reasoning within description logics—is also highlighted.