Dante takes inspiration from rhetorical essays written in Latin, Occitan, Sicilian, and Italian, and from philosophical readings. The main classical rhetorical texts from which he drew information were the "Ars Poetica" by Horace, the "Rhetorica ad Herennium" by an anonymous author, and "De Inventione" by Cicero. About the philosophical works, it is important to know that Dante read not only first hand texts, but also summaries that sometimes were not of the original work, but of an intermediary one. The influence and importance of the contribution of the Sicilian language is emphasized by his assertion that "the first hundred and fifty years of Italian poetry was written in Sicilian".