Within a generation of its founding, Baghdad became a hub of learning and commerce. The city flourished into an unrivaled intellectual center of science, medicine, philosophy, and education, especially with the Abbasid translation movement began under the second caliph Al-Mansur and thrived under the seventh caliph Al-Ma'mun. ''Baytul-Hikmah'' or the "House of Wisdom" was among the most well known academies, and had the largest selection of books in the world by the middle of the 9th century. Notable scholars based in Baghdad during this time include translator Hunayn ibn Ishaq, mathematician al-Khwarizmi, and philosopher Al-Kindi. Although Arabic was used as the international language of science, the scholarship involved not only Arabs, but also Persians, Syriacs, Nestorians, Jews, Arab Christians, and people from other ethnic and religious groups native to the region. These are considered among the fundamental elements that contributed to the flourishing of scholarship in the Medieval Islamic world. Baghdad was also a significant center of Islamic religious learning, with Al-Jahiz contributing to the formation of Mu'tazili theology, as well as Al-Tabari culminating in the scholarship on the Quranic exegesis. Baghdad was likely the largest city in the world from shortly after its foundation until the 930s, when it tied with Córdoba.