The Principle of Dependent Choice is shown to be equivalent to: the Baire Category Theorem for Čech-complete spaces (or for complete metric spaces); the existence theorem for generic sets of forcing conditions; and a proof-theoretic principle that abstracts the "Henkin method" of proving deductive completeness of logical systems. The Rasiowa-Sikorski Lemma is shown to be equivalent to the conjunction of the Ultrafilter Theorem and the Baire Category Theorem for compact Hausdorff spaces.

Paul Cohen’s method of forcing, together with Saul Kripke’s related semantics for modal and intuitionistic logic, has had profound effects on a number of branches of mathematical logic, from set theory and model theory to constructive and categorical logic. Here, I argue that forcing also has a place in traditional Hilbert-style proof theory, where the goal is to formalize portions of ordinary mathematics in restricted axiomatic theories, and study those theories in constructive or syntactic terms. I will discuss the aspects (...) of forcing that are useful in this respect, and some sample applications. The latter include ways of obtaining conservation results for classical and intuitionistic theories, interpreting classical theories in constructive ones, and constructivizing model-theoretic arguments. (shrink)

L’article montre d’abord jusqu’où convergent la dialectique hégélienne de l’Idée et la dialectique lautmanienne des Idées, et ce sur quoi elles se séparent en profondeur : sur la négativité et le statut de la contradiction. Il s’intéresse ensuite à certaines formalisations qui ont été proposées de ces deux dialectiques : celle de Doz et Dubarle pour Hegel dans une extension de la logique booléenne, et celle, récemment esquissée par F. Zalamea en théorie des catégories, pour Lautman. Est montré dans ses (...) grandes lignes comment la traductibilité mutuelle, au niveau technique, peut être établie entre les deux entreprises, la conséquence étant que la divergence spéculative semble rétroactivement gommée, que le négatif semble avoir disparu. À partir de ce paradoxe sont présentées quelques pistes de réflexion sur les enjeux de la démarche de formalisation, entendue comme entreprise de réduction du fossé existant entre le conceptuel et le formel, et de ce qu’elle révèle des rapports entre mathématiques et philosophie.The paper begins by showing what is common to Hegel’s dialectic of the Idea and Lautman’s dialectic of Ideas, and where they diverge deeply : on negativity and the status of contradiction. It then focuses on two different attempts to formalize these dialectics : Doz and Dubarle’s attempt to formalize the Hegelian one in an extension of Boolean logic ; and, more recently, Zalamea’s formalization of the Lautmanian one through category theory. The paper then sketches how, at a technical level, these two attempts can be translated one into the other — which indicates, in turn, that the speculative divergence seems to have disappeared, that the negative has been erased. With reference to this paradox the paper provides the reader with some reflections dealing with the stakes of the formalizing process, seen as way of reducing the gap existing between the conceptual and the definite, and of what it reveals of the relations between mathematics and philosophy. (shrink)

We extend Bishopʼs concept of function spaces to the concept of pre-function spaces. We show that there is an adjunction between the category of neighbourhood spaces and the category of Φ-closed pre-function spaces. We also show that there is an adjunction between the category of uniform spaces and the category of Ψ-closed pre-function spaces.