Abū Yūsuf Yaʿqūb b. Isḥāq al-Kindī (c. 180–250/795–865) flourished in particular in the reign of al-Muʿtaṣim (reigned 218–27/833–42). It is said that he served as tutor to the caliph's son Aḥmad, to whom some of his writings are dedicated. Others are dedicated to the caliph himself. Most are short didactic pieces of strictly limited scope. A few dozen survive, some in Latin or Hebrew translation. Many more titles are recorded by the bibliographers, covering an enormous range of subjects. Al-Kindī wrote on questions of mathematics, logic, physics, psychology, metaphysics and ethics, but also on perfumes, drugs, foods, precious stones, musical instruments, swords, bees and pigeons. He wrote against the false claims of the alchemists, the atomism of the mutakallimūn, the dualism of the Manichaeans, and the trinitarian dogma of the Christians. He supported astrology, calculated the duration of the Arab empire, and speculated on the causes of natural phenomena such as comets, earthquakes, tides or the colour of the sky. He also took an interest in distant countries and ancient nations, collecting information on Socrates (whom he confused with Diogenes the Cynic), the Ḥarranians and the rites of India. A similar range of topics was later covered by al-Kindī's pupil Aḥmad b. al-Ṭayyib al-Sarakhsī, tutor and boon-companion of the caliph al-Muʿtaḍid (reigned 279–89/892–902). No doubt al-Kindī, too, had played the part of a cultured polymath who, wearing his learning lightly, strove to captivate, divert and instruct a courtly public.