David Makinson

We explore ways in which purely qualitative belief change in the AGM tradition throws light on options in the treatment of conditional probability. First, by helping see why it can be useful to go beyond the ratio rule defining conditional from one-place probability. Second, by clarifying what is at stake in different ways of doing that. Third, by suggesting novel forms of conditional probability corresponding to familiar variants of qualitative belief change, and conversely. Likewise, we explain how recent work on the qualitative part of probabilistic inference leads to a very broad class of 'proto-probability' functions.

Summary THIS PAPER has two main purposes. First, to give an exposition of Stenius' approach to the logic of statements of disjunctive permission in such a way as to make manifest its differences with other main lines of attack in the literature. Second, to draw attention to some linguistic phenomena extending beyond the confines of deon‐tic discourse, that appear to reveal the inappropriateness of the other lines described, and to provide some confirmation of an approach, which we shall call the “checklist conception”, that can be seen as a development of that of Stenius. This “checklist conception” also throws light on the behaviour of counterfactual conditionals with disjunctive antecedents.

The interplay of introduction and elimination rules for propositional connectives is often seen as suggesting a distinguished role for intuitionistic logic. We prove three formal results concerning intuitionistic propositional logic that bear on that perspective, and discuss their significance. First, for a range of connectives including both negation and the falsum, there are no classically or intuitionistically correct introduction rules. Second, irrespective of the choice of negation or the falsum as a primitive connective, classical and intuitionistic consequence satisfy exactly the same structural, introduction, and elimination (briefly, elementary) rules. Third, for falsum as primitive only, intuitionistic consequence is the least consequence relation that satisfies all classically correct elementary rules

We investigate introduction and elimination rules for truth-functional connectives, focusing on the general questions of the existence, for a given connective, of at least one such rule that it satisfies, and the uniqueness of a connective with respect to the set of all of them. The answers are straightforward in the context of rules using general set/set sequents of formulae, but rather complex and asymmetric in the restricted (but more often used) context of set/formula sequents, as also in the intermediate set/formula-or-empty context.