SUMMARY It is remarkable how little interest the specialists have taken in John Cassian's De Incarnatione. The principle reason for this is the fact that it gives a misinterpretation of Nestorianism. The study of this book is however interesting, because we find in it the reaction of the West, before this heretic had been condemned at Ephesus. The following problem ought to be examined: What was it that caused Cassian to think that Pelagianism and Nestorianism were the same? The reason for this mistake of Cassian needs to be further examined. Cassian was well informed about Nestorianism: he possessed a dossier, containing sermons and letters written by the patriarch, which had been sent to him from Rome. In this article we shall examine one of the reasons, which may explain the misinterpretation of Cassian's books: the influence which Leporius' Libellus emendationis exercised on Cassian. The sources about Leporianism are vague and contradictory. Leporius' error consisted in laying too much stress upon the independence of Christ's human nature. Cassian followed this heresy attentively, and even had a part in Leporius' conversion; the latter revoked his thesis in his Libellus. When Cassian became aware of Nestorianism, he thought he had discovered „a Leporius redivivus”. It is also possible that the case of Leporius influenced him in his criticism of Nestorianism. This possibility is indicated by the fact that Cassian takes a long quotation from the Libellus as a starting point for his De Incarnatione, and he makes the remark that Nestorius, who followed Leporius in his error, should also folow him in his conversion. The influence of Leporius becomes clear when we see the theses, which according to Cassian, were maintained by Nestorius: the profession of two Christs; the bringing in of a fourth divine person in the Holy Trinity; the idea that Incarnation brought about a humiliation for the divinity; the separation of the two natures into two persons; the declaration that Jesus was only a common saint, who subsequently became God. All these theses are not based upon the words of Nestorius, which Cassian found in his dossier. On the contrary: a lot of these theses prove the contrary of what Nestorius had said in these letters and sermons. But they are all to be found in Leporius' Libellus, though few of them as explicit theses, and others only as points, which he had never defended, but which can be derived logically from Leporianism. It is therefore permissible to conclude with some certainty, that Cassian thought he had found in the Libellus an outline of Nestorianism, one that was clearer than the scanty information he found in his dossier about Nestorianism. Thus as far as the teaching of the Libellus is concerned, its influence on Cassian's opinion of Nestorianism has been proved. Did he also take over the arguments from Holy Scripture? Cassian uses arguments of Holy Scripture nearly ten times more often than Leporius; his work is also much more voluminous. But it is striking that he also used, and in the same sense, all the texts that Leporius availed himself of in his argument, except Jo. Ill, 14 and Ps. XXI, 1. We have shown that there was a serious reason why Cassian could not find a place for these last texts in his refutation of Nestorianism. To sum up, we are inclined to believe that the texts of Holy Scripture in the Libellus helped Cassian, and gave him the material for a wider argumentation against Nestoranism from Holy Scripture. Finally we may ask whether Leporius influenced Cassian in his terminology. Both authors seem to use the current terminology of their time to express the doctrine about Christ. We can only conclude that from a comparison of the terminology used by them both, nothing can be proved either way. All that is certain is that Cassian described Nestorius' doctrine as it appears in Leporius' Libellus.