The fascination exerted by an intellectual product as complex and refined as Aristotelian syllogism has fed, since ancient times, the question of its genesis. In the contemporary debate on the subject, which developed between the end of the nineteenth century and the middle of the twentieth century, the prevailing thesis is that Aristotle developed syllogistics starting from a comparison with the Platonic διαίρεσις, or dichotomous division («διαίρεσις-theory»). With the present study we take a position within the discussion, and, faced with the unsustainability of the many arguments in favor of the «διαίρεσις-theory», as well as the consistency of the objections that can be raised, we retain and present the hypothesis of poor notoriety, advanced in the 1920s by the American philologist Paul Shorey, according to which the antecedent of the inferential scheme of the Stagirite is the method of Ideas outlined by Plato in the Phaedo. In this way we propose to offer a contribution so that the interesting question of the origin of Aristotelian syllogism, substantially fallen into oblivion, is returned to the attention of current research.
Keywords: syllogism, διαίρεσις, Idea, αἰτία, part-whole relationship, intention-extension, cause, explanation, participation.