342. Philosophical Teaching. -- Siger of Brabant assumed the ambitious role of leader of a school of thought. He taught and wrote openly contra praecipuos viros in philosophia Albertum et Thomam,^{1} accusing them of misinterpreting Aristotle. He himself defended the whole Averroïstic credo, as may be seen from the censures, which were directed against him mainly, and from the study of his own works.^{2} Mandonnet describes the close contest between Siger and St. Thomas on the crucial question of the numerical unity of intelligence in the human race; he sketches simultaneously the attack and the defence. Siger's line of exposition in the De Anima Intellectiva is as follows: In addition to the vegetative-sentient soul which "informs" each separate human organism, there exists an intellectual soul, apart and detached from the body by nature, but which effects a temporary union with the latter in order to elicit intellectual thought. This soul is immaterial; and, furthermore, it is unique, because it excludes that which is the very principle of individual or numerical multiplication, namely, matter. This single human soul, moreover, is never completely separated from all human bodies, for in it resides the entire essence of the race: men die, but humanity is immortal.^{3}

In two chapters of the De Anima Intellectiva, and in the De Aeternitate Mundi, Siger proves, in support of his teaching on the eternity of creation, that in terrestrial species, where all the individuals appear by way of "generation," it is impossible to suppose a first generator. It is impossible also to suppose a beginning or an end to the life of immaterial beings. Hence in both departments of being the world is stamped with the seal of eternity.^{4} And therefore the question of immortality, of a future life, does not arise: it is already settled.

The theory of the "two truths" -- the "philosophically true" and the "theologically true" -- is constantly invoked by the philosopher of Brabant; and it covers his daring declarations with a veil through which it is difficult to determine the real state of his mind. Whether he used the double-truth theory as an expression of profane trifling or of a cynical scepticism, his procedure drew from St. Thomas some severe but well-deserved rebukes.^{5} The tone of the refutations in the De Unitate Intellectus is more grave and stern than in any other of the Angelic Doctor's works.

{1} Chartul., i., p. 487.

{2} MANDONNET, op. cit., ch. vi. sqq.

{3} MANDONNET, op. cit., pp. clv, clxxxiii and foll.

{4} Ibid., p. clxxxii.

{5} De Unitate Intellectus, c. vii., and a sermon of St. Thomas. Cf. MANDONNET, p. clxx. There is no end to conjectures as to why Dante places his eulogium of Siger in the mouth of St. Thomas. The following is Baeumker's highly probable explanation (op. cit., pp. 97 sqq.): The poet of the Divina Comedia followed his personal sympathies in dealing with the men of his time and was not sufficiently au courant with academic controversies to take up any deliberately reasoned position on matters philosophical. Just as he makes St. Bonaventure sing the praises of Joachim of Floris, so he makes St. Thomas proclaim the merits of Siger of Brabant. Mandonnet has a different hypothesis (op. cit., ccxciii sqq.).