is not otherwise mentioned in this connection where we should expect it, as for example in the letter of Pope Alexander IV of the year 1256 in which he names three other persons beside William of St. Amour himself as "principales huiusmodi rebellionis et contumaciæ incentores," Mandonnet is undoubtedly correct in maintaining that the first well attested mention of Siger is of the year 1266 in connection with the university troubles.
Mandonnet is able also to connect plausibly Simon Duval's citation of Siger and Bernier de Nivelles before his tribunal for heresy with the condemnation of the former, March 7, 1277, by proving that the date of the summons is not 1278, as Baeumker thought, but October 23, 1277, from the Ms. act of summons as published in Martène-Durand, Thesaurus Anecdotorum.
The main difference between them, however, was in their judgment of Siger's place as a philosopher, and in particular of the character and authorship of the Impossibilia. Beaumker, following Hauréau, expressed the opinion that the Impossibilia is an anonymous refutation of certain theses maintained by Siger, hence all that may be attributed to Siger in this work are the theses and the arguments by which they are defended. The rest, and the more important part of the treatise, belongs to the anonymous author. Understanding the treatise as he did, and having no other works of Siger to go by, Baeumker had only external notices to depend upon in characterizing Siger as an Averroist.
Mandonnet, on the basis of the other works of Siger, in which points of contact are found with the solutions of the theses in the Impossibilia, refuses to accept Baeumker's view and regards the whole treatise as belonging to Siger, and as constituting in its present form a "reportatio" of Siger's formal disputations by one of his auditors or students. He cites other works of the same kind, and holds that these purely dialectic disputations as a matter of exercise in the art of argumentation were very common and formed a part of the intellectual discipline of the schools.
As for Siger himself, Mandonnet has no doubt that he is the chief representative of the Averroists in the University of Paris and the principal opponent of Thomas Aquinas. This he proves not only by the fact that the doctrines condemned in 1270 and 1277 by Etienne Tempier, the Bishop of Paris, many of which are Averroistic in character, affected principally Siger of Brabant and Boethius of Dacia, since they were the only persons punished, and by the fact that the treatise of Thomas Aquinas, De unitate intellectus contra Averroistas,
