The Liber contra collatorem is a treatise written in 432-433 by Prosper of Aquitaine, who refutes the positions taken by a certain “lecturer”, by which John Cassian is meant, author of the Conferences. As an advocate of Augustine and of his doctrine of grace, Prosper has provided in this work most fully his criticism of the theories expressed by the bishop of Hippo’s Southern-French opponents. In order to do this, Prosper built only on excerpts from Conference nr. 13, “On the protection of God”. The first part of this thesis addresses the principal aspects of the work: the purpose of the treatise, the modus operandi, the literary genre, polemical practices, and theological issues. In doing so, this thesis defends the hypothesis that Prosper wrote his treatise in order to assemble a sufficiently argued file to obtain an official condemnation by the bishop of Rome of what has been called “Semipelagianism” and thus the recognition of the authority of the Augustinian doctrine of grace. The second part presents a study of the manuscript tradition of the treatise and of its medieval testimonies, which allows unraveling a rich and complex textual history. Hence, a new edition is justified, the first critical one, correcting and replacing the textus receptus dating from 1711, which is problematic at various points. The edition here presented is provided with a French translation and annotated in order to cast light on the most important or les easily comprehensible passages of the text.