The concept of relevance between classical propositional formulae, defined in terms of letter-sharing, has been around for a long time. But it began to take on a fresh life in the late 1990s when it was reconsidered in the context of the logic of belief change. Two new ideas appeared in independent work of Odinaldo Rodrigues and Rohit Parikh: the relation of relevance was considered modulo the choice of a background belief set, and the belief set was put into a canonical form, called its finest splitting. In the first part of this paper, we recall the ideas of Rodrigues and Parikh, and show that they yield equivalent definitions of what may be called canonical cell/path relevance. The second part presents the main new result of the paper: while the relation of canonical relevance is syntax-independent in the usual sense of the term, it nevertheless remains language-dependent in a deeper sense, as is shown with an example. The final part of the paper turns to questions of application, where we present a new concept of parameter-sensitive relevance that relaxes the Rodrigues/Parikh definition, allowing it to take into account extra-logical sources as well as purely logical ones.From syntactic to canonical cell/path relevance
Logical relevance as a two-place relation between formulaeAttempts to give formal expression to the notion of relevance between propositional formulae go back at least to Belnap [2], who suggested that a necessary, but not sufficient, condition for one formula to be relevant to another is that they share some elementary letter. We call this syntactical relevance. Definition 1.1. Let a, b be formulae of a given propositional logic. They are syntactically relevant to each other iff they share some elementary letter.In the same paper, Belnap went on to propose that relevance of antecedent to consequent should serve as an adequacy condition for any acceptable entailment relation in propositional logic. While classical logic fails syntactic relevance, his subclassical logic E (for 'entailment') satisfies that formal condition, as do a number of other subsystems of classical logic that came to be known as 'relevance logics'.The present paper is not at all concerned with such relevance logics, and we have no desire to weaken the classical one. We are interested in the concept of relevance itself. Our purpose is to see how far the simple idea of letter-sharing may be developed into a well-behaved formal account of relevance in classical propositional contexts, and examine its application to the theory of belief change.Poor behaviour of syntactic relevance. (1) The relation is syntax-dependent. In other words, formulae a, b may be classically equivalent to a , b respectively, and a relevant to b but a not relevant to b . Moreover (2), for any a, b there are a , b to which they are respectively classically equivalent, with a syntactically relevant to b . Example 1.1. For (1), the formula ¬p ∧ (¬p ∨ q) is syntactically relevant to q, but the former is classically equivalent to ¬p which is not relevant to ...