Are Some Modus Ponens Arguments Deductively Invalid?

This article concerns the structure of defeasible arguments like: 'if Bob has red spots, Bob has the measles; Bob has red spots; therefore Bob has the measles.' The issue is whether such arguments have the form of modus ponens or not. Either way there is a problem. If they don't have the form of modus ponens, the common opinion to the contrary taught in leading logic textbooks is wrong. But if they do have the form of modus ponens, doubts are raised about the conventional dogma that all arguments having the form of modus ponens are deductively valid. By carefully examining arguments on both sides of the issue, reasonable doubts are raised about the view that all arguments having a modus ponens form are val i d. University of Winnipeg Resume: On concentre notre attention sur la structure des arguments tels que "Si Bob a des papules rouges, il a la rougeole; Bob a des papules rouges; donc il a la rougeole". Le point en litige est a savoir si de tels arguments ont la formc de modus ponens. S'ils n'ont pas cette formc, alors l'opinion courante exprimee dans les principaux manuels de l ogique est erronee. Mais s'ils ont la forme de modus ponens, on souleve des doutes au sujet du dogme conventionnel selon lequel tout argument ayant cette forme est valide. Un examinant soigne des arguments opposes constituant cette controverse fait soulever des doutes raisonnables sur I'avis que tout argument ayant la forme de modus ponens est valide.