 While the notion of necessary truth-preservation was already quite mature in the Prior Analytics, the conceptual development of the substitutivity criterion is essentially a later contribution of the ancient commentators (Barnes 1990, 2008; Dutilh Novaes 2012a). Recall that Aristotle had not applied the metaphysical notions of form and matter to logical objects such as sentences and arguments in any systematic way; this crucial step was undertaken by the ancient commentators. References to the form and matter of syllogisms are pervasive in their writings, especially in commentaries on the Prior Analytics, from that of Alexander of Aphrodisias (2nd century AD) up to that of Ammonius (6th century AD). The ancient commentators not only distinguished between the form and the matter of syllogisms: they sometimes also suggested (though usually rather obliquely) that the form of an argument is precisely that in virtue of which it is valid and reliable. This would later pave the way for the distinction between formal and material consequences and the idea of validity in virtue of form. Here is an illustrative passage by Alexander of Aphrodisias: