Syllogism as a Combination in Alexander Of Aphrodisias

In the ancient commentary on Aristotelian logic, the Greek word sigma upsilon zeta upsilon gamma iota alpha (syzygia) refers to a pair of propositions that have one or both terms in common. This Greek voice is presented as the genus from which the three deductive rules of Aristotelian logic are specified: conversion (if both terms are common, but change their position), opposition (if both terms are common, but do not change their position) and syllogism (if a single term is common and changes its position). This teaching is not explicit in Aristotle, but it is already known by his main commentator Alexander of Aphrodisias. This article shows that the presence of this teaching leads to describe syllogistic as the activity that seeks the premises that justify a conclusion, which is the task of the dialectic of Aristotle's Topics and explains why Alexander's commentary proposes a dialectical context to discuss syllogistic in Prior Analytics. Finally, on the basis that the use of the term syzygia is attested in authors contemporary and prior to Alexander, such as Cicero, Apuleius and Galen, the article suggests that the elaboration and transmission of this teaching is related to an earlier source, who could be Aristotle's disciple, Theophrastus of Eresus.