or the Trustworthy, the surname of Abu l'Abbas Abdallah, eldest son of the caliph Haroun Alraschid, who was born at Baghdad A.D. 785. By the will of his father, the younger brother Al Amin, as the son of his favourite wife, succeeded to the caliphate, while the elder Almamun was left second in the order of succession. He was at that time governor of Khorassan, and in obedience to his father's will acknowledged the supremacy of his brother. He, however, repaid his good-will with open hatred, and unjust attempts to exclude him from the succession. Almamun was thus forced to consult measures for his own safety and promotion, and caused himself to be proclaimed caliph. After various struggles, his general Thafer, in the year 813, took possession of Baghdad, pursued Al Amin to his retreat, and caused him to be assassinated, so that Almamun remained without a competitor. Various rebellions disturbed the tranquillity of the first years of his reign; but, by his prudent administration and vigorous exertions, these were at length extinguished. Instigated by the advice of his vizier, he soon after raised greater commotions, and exposed his dignity to greater dangers, by countenancing the sect of Ali. He invited to court Imam Rizza, gave him his daughter in marriage, and even declared him his successor in the empire. He assumed the green turban, the colour of the house of Ali, and obliged his courtiers and soldiers to imitate his example. Alarmed at these proceedings, the orthodox Mussulmans, and the house of Abbas, excited a great revolt in Baghdad, and proclaimed Ibrahim, Almamun's uncle, caliph. A civil war was just about to commence, when Fadel the vizier was assassinated, and Rizza died. The people of Baghdad then deposed Ibrahim, and returned to their former allegiance. Taking advantage of Almamun's absence, Thafer seized upon the government of Khorassan, where he founded a dynasty which existed but sixteen years.

Almamun employed the period of tranquillity that followed, in introducing literature into his dominions, and in its improvement; which constitutes the greatest glory of his reign. During the days of his father he discovered an ardent thirst after knowledge, by forming a college in Khorassan, adorned by the most eminent men of various countries; and appointed Mesue, a famous Christian physician of Damascus, for their president. When his father remonstrated against conferring such an honour upon a Christian, he reminded him that the most learned men and most skilful artists in his dominions were Jews and Christians; and added, that he had chosen Mesue as a preceptor in science and useful arts, and not as a teacher of religion. Under his auspices Baghdad became the seat of literature, of private and academical instruction, and the habitation of men of eminence from all quarters. Many valuable books in the Greek, Persian, Chaldean, and Coptic languages, among which were the works of Aristotle and Galen, were translated into the Arabic at his own expense. This caliph himself deemed it an honour to set an example to others of the becoming respect due to mental cultivation, by visiting the schools, and treating the professors with great regard. In mathematics, astronomy, and philosophy, he made a rapid and extensive progress. He was the author of astronomical tables, which, on account of their accuracy, have been much admired. By these various exertions the character of the Saracens was suddenly changed from rudeness and ferocity to politeness and civilisation, while the most powerful and extensive of the European states were involved in ignorance and barbarism. Literature has sustained some irreparable losses from his too great partiality to the Arabic writers, which induced him to destroy the originals of the translated manuscripts. He is represented by the Sonnites, or orthodox Mahometans, as little better than an infidel, because of his attention to philosophy and letters. His conduct, however, shows that he was not sufficiently careful to preserve a philosophical mean between the different religious parties during the time of his administration, as he openly manifested a predilection for the doctrines of the Motazeli, who asserted the free-will of man, and denied the eternity of the Koran. Some allege that, on account of the murmurs which arose against him, he was induced to exhibit too great a zeal, by establishing a kind of inquisition, to compel all his subjects to profess Islamism. The experiment, however, soon terminated in the better and juster expedient of universal toleration; and it is abundantly evident that the Christians in his dominions never felt the power of his inquisition.

The public transactions of his reign are in themselves important. In the years 822 he sent a body of his troops to the assistance of Thomas, a Greek, who made war on Michael the Stammerer, the emperor of Constantinople, and besieged his capital. This expedition, which on the part of the caliph seems to have been founded in justice, proved unsuccessful; Thomas was taken prisoner, and suffered death. In the years 829 and 830 he commenced hostilities upon the Greeks, rendered himself master of many places, and carried devastation into their territories. He was successful in suppressing a revolt in Egypt in the year 831. In this country he was led to discover a treasure buried under Almanack, two columns by Merwan, the last caliph of the house of Om-mlah. In repairing a decayed milius or measuring pillar, and erecting a new one for determining the graduation of the increase of the Nile, Almamun displayed his love of science. In the year 833 he again visited Egypt; on his return he penetrated into the territories of the Greek emperor, even into Cilicia. Returning home, he encamped on the banks of a river, and, excited by thirst, drank too freely of the water, and at the same time indulged himself immoderately in eating a particular kind of dates, which brought on a complaint in his stomach, and reduced him to the most imminent danger. Sensible of his approaching dissolution, he sent letters into all the provinces, declaring his brother Motassem his successor, and then patiently waited the event. After a tedious struggle under the pressure of his disease, and while uttering this ejaculation, "O thou who never diest, have mercy on me, a dying man!" he expired at the age of 48 or 49 years, after a reign of 27 years and some months.