 A major impetus for science in the Islamic world was the patronage of the Abbasid caliphate (758–1258), centered in Baghdad. Early Abbasid rulers, such as Harun al-Rashid (ruled 786–809) and his successor Abū Jaʿfar Abdullāh al-Ma’mūn (ruled 813–833), were significant patrons of science. The former founded the Bayt al-Hikma (House of Wisdom), which commissioned translations of major works by Aristotle, Galen, and many Persian and Indian scholars into Arabic. It was cosmopolitan in its outlook, employing astronomers, mathematicians, and physicians from abroad, including Indian mathematicians and Nestorian (Christian) astronomers. Throughout the Islamic world, public libraries attached to mosques provided access to a vast compendium of knowledge, which spread Islam, Greek philosophy, and science. The use of a common language (Arabic), as well as common religious and political institutions and flourishing trade relations encouraged the spread of scientific ideas throughout the Islamic world. Some of this transmission was informal, e.g., correspondence between like-minded people (see Dhanani 2002), some formal, e.g., in hospitals where students learned about medicine in a practical, master-apprentice setting, and in astronomical observatories and academies. The decline and fall of the Abbasid caliphate dealt a blow to science in the Islamic world, but it remains unclear why it ultimately stagnated, and why it did not experience something analogous to the scientific revolution in Western Europe. Note, the decline of science in the Islamic world should not be generalized to other fields, such as philosophy and philosophical theology, which continued to flourish after the Abbasid caliphate fell.