al- Mamun (Abu al-Abbas Abd Allah al-Mamun) (mämōōn´), 786–833, 7th Abbasid caliph (813–33); son of Harun ar-Rashid. He succeeded his brother al-Amin after a bitter civil war, but was unable to enter Baghdad until 819. He was himself one of the Mutazilites, holding that the Qur'an was created in time, i.e., that it was not an uncreated eternal existent. He persecuted the orthodox bitterly. Al-Mamun's reign was one of great cultural achievement, and he was especially interested in the work of scientists, particularly of those who knew Greek. He established (830) in Baghdad the House of Wisdom, an institution that translated Greek works into Arabic.