Les articles regroupés dans ce numéro spécial, à l'exception de celui de P. Roeper et H. Leblanc «De A et B, de leur indépendance logique, et de ce qu'ils n'ont aucun contenu factuel commun», ont tous pour origine un colloque tenu les 25 et 26 mars 1994 à l'Université du Québec à Montréal. C'était pour souligner la contribution sans égale faite par un Québécois au développement de la logique qu'à l'occasion du soixante-dixième anniversaire de naissance du Professeur Hugues Leblanc est née l'idée d'organiser ce colloque. Parce que c'est en partie sous l'impulsion fournie par le travail de H. Leblanc qu'ont été développées des sémantiques et des logiques non standard (logiques libres, avec ou sans identité; sémantique en termes de valeurs de vérité; et sémantique probabiliste entre autres), ce colloque portait le titre «Logiques et sémantiques non classiques».

Professor Roeper adresses a large question, whether probabilistic semantics is a kind of semantics at all. Happily, he does this via an exploration of a specific issue on which he and Professor Leblanc have done important work. That is the issue of how the relationship of logical consequence can be characterized as a relation denned in terms of probability. Let us follow him in calling a relevant relationship of the latter sort the degree of implication, and follow Professor Roeper on his quest for a satisfactory one.

This paper can be read as an attempt at providing philosophical foundations to linear logic. The only plausible form of philosophical antirealism deals with practical feasibility rather than with effectivity in principle. The very notion of recognizability is ambiguous, audit has to be considered from a stricter perspective than currently done. The intuitionistic assertability conditions are to be reinforced. This change requires a move towards a frame in which the circumstances of the application of a logical rule can be specified. Gentzen's sequential calculi provide such a frame, when some structural rules have been removed or limited.

Personal probability is now a familiar subject in epistemology, together with such more venerable notions as knowledge and belief. But there are severe strains between probability and belief; if either is taken as the more basic, the other may suffer. After explaining the difficulties of attempts to accommodate both, I shall propose a unified account which takes conditional personal probability as basic. Full belief is therefore a defined, derivative notion. Yet we will still be able to picture opinion as follows: my subjective probability is only a grading of the possibilities left open by my beliefs. My conditional probabilities generally derive — in a sense to be explicated — from the strongest belief I can maintain when admitting the relevant condition.

As van Fraassen pointed out in his opening remarks, Henry Kyburg's lottery paradox has long been known to raise difficulties in attempts to represent full belief as a probability greater than or equal to p, where p is some number less than 1. Recently, Patrick Maher has pointed out that to identify full belief with probability equal to 1 presents similar difficulties. In his paper, van Fraassen investigates ways of representing full belief by personal probability which avoid the difficulties raised by Maher's measure-theoretic version of the lottery paradox. Van Fraassen's more subtle representation dissolves the simple identification of full belief with maximal personal probability. His investigation exploits the richer resources for representing opinion provided by taking conditional, rather than unconditional, personal probability as fundamental. It has interesting implications for equivalent alternative approaches based on non-Archimedean probability, as well as for equivalent approaches in which assumption contexts representing full belief relative to suppositions are taken as fundamental.

Since I wrote my doctoral dissertation lo! these forty-seven years ago under Professor Quine, my colleague Alain Voizard thought it fitting that I comment on the paper just delivered. The invitation flattered me, I confess, as did Professor Quine's choice of a subject: free logic, a logic of which I was one of the numerous fathers thirty-five years ago. So, it is in my capacity as a commentator that I address you now, saving for tomorrow afternoon further thoughts prompted by this colloquium.

The logical independence of two statements is tantamount to their probabilistic independence, the latter understood in a sense that derives from stochastic independence. And analogous logical and probabilistic senses of having the same factual content similarly coincide. These results are extended to notions of non-symmetrical independence and independence among more than two statements.

Certains philosophes se sentent plus à l'aise dans l'essai ou l'article que dans le livre. Il arrive que même quand leurs essais sont des classiques, discutes abondamment depuis des années, la parution en volume tarde à venir. Mais quand elle vient, le recueil s'impose d'emblée comme un ouvrage majeur, et révèle la cohérence de la démarche de l'auteur et le caractère impressionnant de ses contributions. Ce livre de Ruth Barcan Marcus est de ceux-là.

Imagine a model of the universe that, if true and known to be true, would solve the following philosophical problems: the direction and flow of time, an ontology for laws of nature, the interpretation of quantum mechanics, the interpretation of probability, a semantics for counterfactuals, trans-world and trans-temporal identity, essentialism and natural kinds, and free will and responsibility. The successful solution to these problems would convince most of us that we should, at the very least, give this model serious consideration. This is the argument of Storrs McCall's A Model of the Universe: Space-Time, Probability, and Decision. Assume that the branched-tree model of the universe is true, and all these problems are solved. Thus, the branched-tree model should at least be a plausible candidate for the true model of the universe.

The flurry of debates on mental causation in recent years has largely been occasioned by Donald Davidson's original and controversial views on the role of mind in the causation and explanation of behaviour. In his classic 1963 paper, “Actions, Reasons, and Causes,” Davidson argued, against the prevailing opinion of the Ryleans and later-Wittgensteineans, that in order to be genuinely explanatory of human behaviour, reasons must be causes; and in his equally influential and far more controversial 1970 paper, “Mental Events,” he undertook to show how reasons can be causes—how it is possible for our beliefs, desire, intentions, and the like, in terms of which we “rationalize” our behaviour, to be at the same time causes of our behaviour. While the basic thesis of the 1963 paper was widely accepted, setting a trend for much of the work on action theory for the following three decades, the account of the causal efficacy of mind in “Mental Events” generated a great deal of controversy. The debates continue, involving scores of participants but seldom Davidson himself. What makes the present collection of previously unpublished papers particularly interesting is that it contains Davidson's own reply to some of his most notable critics, together with their rejoinders; so we now have a contemporary perspective on the mental causation debate that Davidson initiated, unwittingly, a quarter of a century ago.