ʿAlī al-Riḍā ibn Mūsā al-Kāẓim ibn Jaʿfar al-Ṣādiq ibn Muḥammad al-Bāqir ibn ʿAlī Zayn al-ʿĀbidīn ibn al-Ḥusayn the martyr, grandson of ʿAlī ibn Abī Ṭālib, and he cast off the Abbasid black and put on green, as we have already mentioned (1). This infuriated the Abbasids of Baghdad and elsewhere, so they deposed al-Maʾmun and installed Ibrāhīm ibn al-Mahdī in his stead, as related above. Al-Maʾmun later overcame them, his authority over the caliphate was re-established after the death of ʿAlī al-Riḍā in Ṭūs, and he pardoned his uncle Ibrāhīm ibn al-Mahdī, as has been set out in detail in its proper place (2).
As for his espousal of the Muʿtazilite doctrine, he kept company with a number of men—among them Bishr ibn Ghayāth al-Marīsī—and adopted that erroneous creed from them. He loved knowledge, but his discernment was not penetrating; accordingly, falsehood found a way into him and gained currency with him. He propagated it and compelled the people to embrace it, and that occurred toward the end of his life and the decline of his reign.
Abū Bakr ibn Abī al-Dunyā said: “Al-Maʾmun was fair-skinned, of medium height, handsome of face; grey had begun to streak his hair, and a slight sallowness tinged his complexion. He was large-eyed, with a long, fine beard, a narrow forehead, and a mole on his cheek. His mother was an umm walad (slave-concubine) named Marājil” (3).
Al-Khaṭīb al-Baghdādī narrated from al-Qāsim ibn Muḥammad ibn ʿAbbād who said, “No caliph memorised the Quran except ʿUthmān ibn ʿAffān and al-Maʾmun” (4); this is exceedingly strange (5).
They also report that in the month of Ramaḍān he would complete thirty-three full recitations of the Quran. On one occasion he sat to dictate hadith; Chief Judge Ibn Aktham and others gathered around him, and he dictated to them from memory thirty hadiths. He possessed insight into many sciences—jurisprudence, medicine, poetry, the laws of inheritance, dialectical theology, grammar, Arabic language and lexicography, and astronomy. The astronomical tables known as the al-Maʾmunī zīj (6) are ascribed to him. He measured the length of one degree on the plain of Sinjār, and his result differed from that of the ancient scholars.
Ibn ʿAsākir (7) relates that al-Maʾmun once sat to hear petitions while princes and scholars were in attendance. A woman came to complain, saying that her brother had died leaving six hundred dinars, yet only one dinar had reached her. He replied to her instantly: “Your full share has reached you; it is as though your brother left two daughters (8), a mother, a wife, twelve brothers, and a sister …”