One often thinks that the Latin Ars contains a so-called ‘third part’, inherited from Stoic linguistic analysis, on the vices and virtues of speech (vitia virtutesque orationis). Contrary to this received opinion, well estab lished in the history of grammar, we propose an evolutionary interpretation predicated on the study of texts as ensembles and not as collections of details. It results that Stoic doctrine is not compatible with the general orientation of the ‘third part’. The structure of this ‘third part’ consists, moreover, of anomalies, which are explained if one assumes that there has been a secondary extension of theoretical grammar from its original domain – correctness – to another ‘property of discourse’, namely ornamentation. Evidence from Quintilian shows how this extension has been able to occur under the twofold influence of rhetoric and literary criticism.