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La descente infinie, l’induction transfinie et le tiers exclu

Published online by Cambridge University Press:  01 March 2009

Yvon Gauthier*
Affiliation:
Université de Montréal

Abstract

ABSTRACT: It is argued that the equivalence, which is usually postulated to hold between infinite descent and transfinite induction in the foundations of arithmetic uses the law of excluded middle through the use of a double negation on the infinite set of natural numbers and therefore cannot be admitted in intuitionistic logic and mathematics, and a fortiori in more radical constructivist foundational schemes. Moreover it is shown that the infinite descent used in Dedekind-Peano arithmetic does not correspond to the infinite descent of classical Fermatian arithmetic or number theory. However, from the point of view of classical logic, the principles of complete induction and transfinite induction, the least number principle and infinite descent are all equivalent. We find here a focal point for a foundational critique that aims for a a clarification of philosophical options in the foundations of logic and mathematics.

RÉSUMÉ : Cet article propose que l’équivalence postulée entre la descente infinie et l’induction transfinie dans les fondements de l’arithmétique fait intervenir le principe du tiers exclu par la double négation sur l’ensemble infini des nombres naturels et ne saurait donc être admissible du point de vue de la logique et des mathématiques intuitionnistes. Si, par ailleurs, on adopte le point de vue de la logique classique, les principes de l’induction complète, de l’induction transfinie, du plus petit nombre et de la descente infinie sont tous équivalents; pourtant, la descente infinie en jeu dans l’arithmétique ensembliste de Dedekind-Peano ne correspond pas à la descente infinie de Fermat en théorie des nombres et en arithmétique classique de Gauss jusqu’à nos jours. C’est là le point d’ancrage d’une critique fondationnelle qui cherche à mieux définir les options philosophiques dans les fondements de la logique et des mathématiques.

Type
Articles
Copyright
Copyright © Canadian Philosophical Association 2009

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