  Beginning in the tenth century, Islamic philosophers such as Alfarabi (ca. 870–950), Avicenna (980–1037), and Averroes (ca.  1126–98), produced commentaries, but their impact in the West was felt only after the period of translations from Arabic into Latin which began in Spain around the middle of the twelfth century. This process introduced many technical terms that came to have considerable impact on philosophical and theological discussions. Also worth mentioning are the Liber sex principiorum, an anonymous work traditionally ascribed to Gilbert of Poitiers (1085/90–1154) that sought to expand Aristotle's brief comments on the last six categories, and Peter Abelard (1079–1142), who discussed categories in his work (see Marenbon 1997).