professor of philosophy at the University of Paris and a leading representative of the school of radical, or heterodox, Aristotelianism, which arose in Paris when Latin translations of Greek and Arabic works in philosophy introduced new material to masters in the faculty of arts.

Beginning about 1260 Siger and some of his colleagues inaugurated purely rational lectures that reinterpreted works of Aristotle without regard for established teachings of the church, which had blended orthodox Aristotelianism with Christian faith. In addition to Aristotle, Siger's sources include such philosophers as Proclus (410–485), Avicenna (980–1037), Averroës (1126–98), and Thomas Aquinas (1225?–74).

From 1266, when his name first appears, to 1276, Siger was prominent in the disputes at Paris over Aristotelianism. Bonaventure, the minister general of the Order of Friars Minor, and Aquinas, head of the Dominicans, both attacked Siger's teachings. In 1270 the bishop of Paris, Étienne Tempier, condemned 13 errors in the teaching of Siger and his partisans. Six years later the inquisitor of the Roman Catholic Church in France summoned Siger and two others suspected of heterodoxy, but they fled to Italy, where they probably entered an appeal before the papal tribunal. A few months later, in March 1277, Tempier announced condemnation of 219 more propositions. Siger is believed to have been restricted to the company of a cleric, for he was stabbed at Orvieto by his cleric, who had gone mad, and he died during Martin IV's pontificate, sometime before Nov. 10, 1284. Dante, in the Divine Comedy, put Siger in the Heaven of Light in the brilliant company of 12 illustrious souls.

Siger's written works gradually came to light, and 14 authentic works and 6 probably authentic commentaries on Aristotle were known by the mid-20th century. Among them are Quaestiones in metaphysicam, Impossibilia (six exercises in sophistry), and Tractatus de anima intellectiva (“Treatise on the Intellectual Soul”). The last discusses his basic belief that there is only one “intellectual” soul for mankind and thus one will. Although this soul is eternal, individual human beings are not immortal. This view, though not lucidly expressed, suggests Siger's disregard for doctrines of the church and his emphasis on maintaining the autonomy of philosophy as a self-sufficient discipline.