The early Muslim compilations of knowledge in the Middle Ages included many comprehensive works. Around year 960, the Brethren of Purity of Basra were engaged in their "Encyclopedia of the Brethren of Purity". Notable works include Abu Bakr al-Razi's encyclopedia of science, the Mutazilite Al-Kindi's prolific output of 270 books, and Ibn Sina's medical encyclopedia, which was a standard reference work for centuries. Also notable are works of universal history (or sociology) from Asharites, al-Tabri, al-Masudi, Tabari's "History of the Prophets and Kings", Ibn Rustah, al-Athir, and Ibn Khaldun, whose "Muqadimmah" contains cautions regarding trust in written records that remain wholly applicable today.