Al-Kindi

Abū Yūsuf Yaʻqūb ibn ʼIsḥāq aṣ-Ṣabbāḥ al-Kindī was an Arab polymath who was active as a philosopher, mathematician, physician, and music theorist. Al-Kindi was the first of the Islamic peripatetic philosophers, and is hailed as the "father of Arab philosophy".
Al-Kindi was born in Kufa and educated in Baghdad. He became a prominent figure in the House of Wisdom, and a number of Abbasid Caliphs appointed him to oversee the translation of Greek scientific and philosophical texts into the Arabic language. This contact with "the philosophy of the ancients" had a profound effect on him, as he synthesized, adapted and promoted Hellenistic and Peripatetic philosophy in the Muslim world. He subsequently wrote hundreds of original treatises of his own on a range of subjects ranging from metaphysics, ethics, logic and psychology, to medicine, pharmacology, mathematics, astronomy, astrology and optics, and further afield to more practical topics like perfumes, swords, jewels, glass, dyes, zoology, tides, mirrors, meteorology and earthquakes.
In the field of mathematics, al-Kindi played an important role in introducing Hindu numerals to the Islamic world, and their further development into Arabic numerals along with al-Khwarizmi which eventually was adopted by the rest of the world. Al-Kindi was also one of the fathers of cryptography. Building on the work of al-Khalil, Al-Kindi's book entitled Manuscript on Deciphering Cryptographic Messages gave rise to the birth of cryptanalysis, was the earliest known use of statistical inference, and introduced several new methods of breaking ciphers, notably frequency analysis. He was able to create a scale that would enable doctors to gauge the effectiveness of their medication by combining his knowledge of mathematics and medicine.
The central theme underpinning al-Kindi's philosophical writings is the compatibility between philosophy and other "orthodox" Islamic sciences, particularly theology, and many of his works deal with subjects that theology had an immediate interest in. These include the nature of God, the soul and prophetic knowledge.

Early life

Al-Kindi was born in Kufa to an aristocratic family of the Arabian tribe of the Kinda, descended from the chieftain al-Ash'ath ibn Qays, a contemporary of Muhammad. The family belonged to the most prominent families of the tribal nobility of Kufa in the early Islamic period, until it lost much of its power following the revolt of Abd al-Rahman ibn Muhammad ibn al-Ash'ath. His father Ishaq was the governor of Basra and al-Kindi received his preliminary education there. He later went to complete his studies in Baghdad, where he was patronized by the Abbasid caliphs al-Ma'mun and al-Mu'tasim. On account of his learning and aptitude for study, al-Ma'mun appointed him to the House of Wisdom, a recently established center for the translation of Greek philosophical and scientific texts, in Baghdad. He was also well known for his beautiful calligraphy, and at one point was employed as a calligrapher by Caliph al-Mutawakkil.
When al-Ma'mun died, his brother, al-Mu'tasim became caliph. Al-Kindi's position would be enhanced under al-Mu'tasim, who appointed him as a tutor to his son. But on the accession of al-Wathiq, and especially of al-Mutawakkil, al-Kindi's star waned. There are various theories concerning this: some attribute al-Kindi's downfall to scholarly rivalries at the House of Wisdom; others refer to al-Mutawakkil's often violent persecution of unorthodox Muslims ; at one point al-Kindi was beaten and his library temporarily confiscated. Henry Corbin, an authority on Islamic studies, says that in 873, al-Kindi died "a lonely man", in Baghdad during the reign of al-Mu'tamid.
After his death, al-Kindi's philosophical works quickly fell into obscurity; many were lost even to later Islamic scholars and historians. Felix Klein-Franke suggests several reasons for this: aside from the militant orthodoxy of al-Mutawakkil, the Mongols also destroyed countless libraries during their invasion of Persia and Mesopotamia. However, he says the most probable cause of this was that his writings never found popularity amongst subsequent influential philosophers such as al-Farabi and Avicenna, who ultimately overshadowed him. His philosophical career peaked under al-Mu'tasim, to whom al-Kindi dedicated his most famous work, On First Philosophy, and whose son Ahmad was tutored by al-Kindi.

Accomplishments

According to Arab bibliographer Ibn al-Nadim, al-Kindi wrote at least two hundred and sixty books, contributing heavily to geometry, medicine and philosophy, logic, and physics. Although most of his books have been lost over the centuries, a few have survived in the form of Latin translations by Gerard of Cremona, and others have been rediscovered in Arabic manuscripts; most importantly, twenty-four of his lost works were located in the mid-twentieth century in a Turkish library.

Philosophy

His greatest contribution to the development of Islamic philosophy was his efforts to make Greek thought both accessible and acceptable to a Muslim audience. Al-Kindi carried out this mission from the House of Wisdom, an institute of translation and learning patronized by the Abbasid Caliphs, in Baghdad. As well as translating many important texts, much of what was to become standard Arabic philosophical vocabulary originated with al-Kindi; indeed, if it had not been for him, the work of philosophers like al-Farabi, Avicenna, and al-Ghazali might not have been possible.
In his writings, one of al-Kindi's central concerns was to demonstrate the compatibility between philosophy and natural theology on the one hand, and revealed or speculative theology on the other. Despite this, he did make clear that he believed revelation was a superior source of knowledge to reason because it guaranteed matters of faith that reason could not uncover. And while his philosophical approach was not always original, and was even considered clumsy by later thinkers, he successfully incorporated Aristotelian and neo-Platonist thought into an Islamic philosophical framework. This was an important factor in the introduction and popularization of Greek philosophy in the Muslim intellectual world.

Astronomy

Al-Kindi took his view of the Solar System from Ptolemy, who placed the Earth at the centre of a series of concentric spheres, in which the known heavenly bodies are embedded. In one of his treatises on the subject, he says that these bodies are rational entities, whose circular motion is in obedience to and worship of God. Their role, al-Kindi believes, is to act as instruments for divine providence. He furnishes empirical evidence as proof for this assertion; different seasons are marked by particular arrangements of the planets and stars ; the appearance and manner of people varies according to the arrangement of heavenly bodies situated above their homeland.
However, he is ambiguous when it comes to the actual process by which the heavenly bodies affect the material world. One theory he posits in his works is from Aristotle, who conceived that the movement of these bodies causes friction in the sub-lunar region, which stirs up the primary elements of earth, fire, air and water, and these combine to produce everything in the material world. An alternative view found in the treatise On Rays is that the planets exercise their influence in straight lines; but this treatise, written by a Latin author, probably around the middle of the 13th century, is apocryphal. In each of these, two fundamentally different views of physical interaction are presented; action by contact and action at a distance. This dichotomy is duplicated in his writings on optics.
Some of the notable astrological works by al-Kindi include:

The Book of the Judgement of the Stars, including The Forty Chapters, on questions and elections.
On the Stellar Rays
Several epistles on weather and meteorology, including De mutatione temporum,.
Treatise on the Judgement of Eclipses.
Treatise on the Dominion of the Arabs and its Duration.
The Choices of Days.
On the Revolutions of the Years.
De Signis Astronomiae Applicitis as Mediciam 'On the Signs of Astronomy as applied to Medicine'
Treatise on the Spirituality of the Planets.
Optics

Al-Kindi was the first major writer on optics since antiquity. Roger Bacon placed him in the first rank after Ptolemy as a writer on the topic. In the apocryphal work known as De radiis stellarum, is developed the theory "that everything in the world... emits rays in every direction, which fill the whole world." This theory of the active power of rays had an influence on later scholars such as Ibn al-Haytham, Robert Grosseteste and Roger Bacon.
Two major theories of optics appear in the writings of al-Kindi: Aristotelian and Euclidean. Aristotle had believed that in order for the eye to perceive an object, both the eye and the object must be in contact with a transparent medium that is filled with light. When these criteria are met, the "sensible form" of the object is transmitted through the medium to the eye. On the other hand, Euclid proposed that vision occurred in straight lines when "rays" from the eye reached an illuminated object and were reflected back. As with his theories on Astrology, the dichotomy of contact and distance is present in al-Kindi's writings on this subject as well.
The factor which al-Kindi relied upon to determine which of these theories was most correct was how adequately each one explained the experience of seeing. For example, Aristotle's theory was unable to account for why the angle at which an individual sees an object affects his perception of it. For example, why a circle viewed from the side will appear as a line. According to Aristotle, the complete sensible form of a circle should be transmitted to the eye and it should appear as a circle. On the other hand, Euclidean optics provided a geometric model that was able to account for this, as well as the length of shadows and reflections in mirrors, because Euclid believed that the visual "rays" could only travel in straight lines. For this reason, al-Kindi considered the latter preponderant.
Al-Kindi's primary optical treatise "De aspectibus" was later translated into Latin. This work, along with Alhazen's Optics and the Arabic translations of Ptolemy and Euclid's Optics, were the main Arabic texts to affect the development of optical investigations in Europe, most notably those of Robert Grosseteste, Vitello and Roger Bacon.