1 Splitting Logics Walter Carnielli and Marcelo E. Coniglio abstract. This paper addresses the question of factoring a logic into families of (generally simpler) components, estimating the top– down perspective, splitting, versus the bottom–up, splicing. Three methods are carefully analyzed and compared: possible–translations semantics, nondeterministic semantics and plain fibring (joint with its particularization, direct union of matrices). The possibilities of inter–definability between these methods are also examined. Finally, applications to some well–known logic systems are given and their significance evaluated. 1 Splitting logics, splicing logics and their use One of fundamental questions in the philosophy of logic, “Why there are so many logics instead of just one?” (or even, instead of none), is naturally counterposed by another: If there are indeed many logics, are they excluding alternatives, or are they compatible? Is it possible to combine them into coherent systems, with the purpose of using them in applications and of taking profit of this composionality capacity to better understand logics? And if we can compose, why not decompose logics? One of the first, and one of the most general, approaches for the ques- tion of combining logics is the concept of fibring introduced by D. Gabbay in [Gabbay, 1996]. Fibring is able to combine logics creating new and ex- pressive systems, in the direction of what we call splicing logics. The other direction is called splitting logics. Though, as we shall ar- gue, there is no essential distinction between splicing and splitting, there are important differences with respect to the aims one may have in mind. Splitting as a process for investigating logics has been under–appreciated, and we intend to stress here some results and some views that we believe to be of interest for the sake of splitting in the trade of combining logics. 1 1 The process tags “splicing” and “splitting” logics were introduced in [Carnielli and Coniglio, 1999]. As a noun, “splitting” is also used in the literature in a completely different sense, viz., to designate a “logic that splits a class”, as e.g. in W.J. Blok, “On the degree of incompleteness of modal logics” (abstract). Bulletin of the Section of Logic of the Polish Academy of Sciences, 7(4):167-175, December 1978. ¡book title¿, 1–26. c  ¡year¿, the author.