his time,
A.H. 204 (August 819), the real reign of Mamun began, freed as he now
was from the tutelage of Fadl.

When welcoming Tahir, Mamun bade him ask for any reward he might desire.
Tahir, fearing lest the caliph, not being able to endure the sight of
the murderer of his brother, should change his mind towards him,
contrived to get himself appointed governor of Khorasan. Like most of
the great Moslem generals, Tahir, it is said, had conceived the project
of creating an independent kingdom for himself. His death, A.H. 207
(A.D. 822), prevented its realization; but as his descendants succeeded
him one after the other in the post of governor, he may be said in
reality to have founded a dynasty in Khorasan. His son Abdallah b. Tahir
was a special favourite of Mamun, He brought Nasr b. Shabath to
subjection in Mesopotamia, and overcame by great ability a very
dangerous rebellion in Egypt. When he returned thence, the caliph gave
him the choice between the government of Khorasan and that of the
northern provinces, where he would have to combat Babak the Khorramite.
Abdallah chose the former (see below, S 8).

The pseudo-caliph, Ibrahim, who, since Mamun's entry into Bagdad, had
led a wandering life, was eventually arrested. But Mamun generously
pardoned him, as well as Fadl b. Rabi', the chief promoter of the
terrible civil war which had so lately shaken the empire. After that
time, Ibrahim lived peacefully at the court, cultivating the arts of
singing and music.

Tranquillity being now everywhere re-established, Mamun gave himself up
to science and literature. He caused works on mathematics, astronomy,
medicine and philosophy to be translated from the Greek, and founded in
Bagdad a kind of academy, called the "House of Science," with a library
and an observatory. It was also by his orders that two learned
mathematicians undertook the measurement of a degree of the earth's
circumference. Mamun interested himself too in questions of religious
dogma. He had embraced the Motazilite doctrine about free will and
predestination, and was in particular shocked at the opinion which had
spread among the Moslem doctors that the Koran was the uncreated word of
God. In the year 212 (A.D. 827) he published an edict by which the
Motazilite (Mu'tazilite) doctrine was declared to be the religion of the
state, the orthodox faith condemned as heretical. At the same time he
ordered all his subjects to honour Ali as the best creature of God