 In the 12th century CE, during the so called “period of decline” of philosophical activities, the most notable figure was the philosopher-mystic, Shaykh Shihāb al-Dīn Suhrawardī, the founder of the School of Illumination (Ishrāq). Suhrawardī, who was a Peripatetic, wrote four major works, primarily in the Aristotelian tradition. He also wrote a number of works in a peculiar esoteric tradition, using what he called “the language of illumination” (lisān al-ishrāq). In his illuminationist writings, Suhrawardī relies on Neoplatonism but replaces such concepts as being and existence with light and illumination, thus offering what can be called a gnostic-illuminationist version of Avicennian philosophy (Aminrazavi 2003). He offers his doctrine of illumination in his major work, The Philosophy of Illumination (Ḥikmat al-ishrāq).