Al-Mansur, the second Abbasid caliph, chose a site on the Tigris in 762 CE and laid his new capital out as a perfect circle, walls and palace concentric around the caliphal mosque at its center, a design meant to embody both defensibility and cosmological order. He called it Madinat al-Salam, though the older Aramaic name of a village on the site, Baghdad, outlasted the official one in common use. Within decades the city had outgrown its round walls to become one of the largest cities in the world.
Its most consequential institution was the House of Wisdom, Bayt al-Hikma, which flourished under Harun al-Rashid and especially his son al-Ma'mun in the early ninth century as a library and center of translation. Scholars there rendered Greek philosophical and scientific works, often by way of earlier Syriac translations, into Arabic: Hunayn ibn Ishaq, a Nestorian Christian physician, led much of the medical and Galenic corpus; Thabit ibn Qurra, from the star-worshipping Harranian community that claimed Quranic protection under the label "Sabian," carried mathematics and astronomy; Jewish scholars and physicians worked alongside them. Al-Kindi, remembered as the first great philosopher writing in Arabic, was active in this same milieu, synthesizing Greek philosophy with Islamic theology.