Introduction: The Arabic term al-„aql / ―intelligence/understanding/ reason‖ is one among half-a-dozen of the most important concepts occurring throughout Islamic experience and thought. From the beginning of the Islamic era, it had been an opaque term, and Muslim scholars did not always agree that „aql was univocal in meaning. In its early Islamic unfolding the concept of „aql comprised the intersection of primarily Arab and Qur‘anic as well as Biblicic components with Hellenic and Iranian traditions. „Aql became the carrier of multiple overlapping or diverging meanings, if not already before Islam among the old Arabs; it assumed particular significances in ethics, humanistic studies (adab), prosody and rhetoric, law, theology, philosophy, as well as in spiritual and metaphysical speculations.1 A review of the Islamic understanding of ‗reason‘ and ‗rationality‘ would have to deal with the chief disciplines wherein rationality played an extensive role: legal theory (usul al-fiqh), speculative theology (kalam), philosophy (falsafah) and rational spirituality (hikmah & „irfan). Attention should also be given to pronounced anti-rationalist features of Traditionalism.