Abu-Yusuf Yaqub ibn-Ishaq al-Kindi (died 873) was the first significant Arabian philosopher to utilize and develop the philosophical conceptions of Greek thought. His work significantly affected the intellectual development of western Europe in the 13th century.
A great achievement of medieval Islamic civilization was the development of a philosophical tradition that preserved and expanded upon many of the important elements of Greek learning and outshone the contemporary philosophical and scientific knowledge of Christian Europe.
Al-Kindi and later Arabian philosophers, such as al-Farabi, Avicenna, Avempace, and Averroës, benefited from the missionary zeal of the Hellenistic Mutazilite movement, which, in the 9th century, encouraged the study of Greek philosophy and logic to combat the Moslem heretics who affirmed either a Gnostic or a Manichaean dualism. Arabian philosophy, as it developed from the 9th to the 12th century, was basically Neoplatonic in structure and incorporated large portions of Aristotelian philosophy, along with elements of Islamic theology that were not directly incompatible with Greek thought.
Generally ignored and sometimes attacked by the more conservative forces in Islamic religion and society, the writings of al-Kindi and his successors made an enormous impact upon scholastic philosophy in the West. Europe initially became acquainted with the majority of the philosophical and scientific concepts of Aristotle through the mediation of Arabian philosophy.
Al-Kindi was born at Al Kufa (in modern Iraq) on the Euphrates in the early years of the 9th century. Most of his life he lived in that region, at Basra, and, eventually, at Baghdad until he died in 873. At that time Baghdad was the cultural and political capital of Islam, and in his studies al-Kindi became acquainted with the philosophy and science of India as well as of Greece. He studied Hindu works that had been translated into Arabic by way of Persian, and Greek works that had been translated by way of Syriac. Al-Kindi undertook some translations, at least from the Syriac into Arabic, and his thought always reflected an eclectic tendency to reconcile different philosophies.
Throughout most of his career al-Kindi held a position as court scholar in Baghdad. He acted as tutor to the son of al-Mutasim (reigned 833-842), dedicating several works to his young pupil. Under the conservative caliph al-Mutawakkil (reigned 847-861), however, al-Kindi was disgraced and his position at court terminated. Little is known of his later life. He seems to have continued his work as a private scholar until his death.
As the first outstanding Arabian scholar, al-Kindi received the honorific title Faylasuf al-Arab (the philosopher of the Arabs). His works are extensive in both number and subject. He composed in Arabic well over 300 treatises and translations. Primarily an encyclopedist, he wrote not only on philosophy and logic but also on arithmetic, geometry, astronomy, meteorology, optics, medicine, politics, and music. Medieval Europe was familiar with only a portion of his writings, the most important being On the Intellect and What Is Understood, On Sleep and Vision, On the Five Essences, Introduction to the Art of Logical Demonstration, The Theory of the Magical Arts, and The Agent in the Proper Sense and in the Metaphorical Sense.
On the Intellect, the most influential of these works, was written to clarify Aristotle's distinction between that portion of the intellect that receives knowledge (the possible intellect) and that portion of the intellect that causes knowledge by reproducing intelligible objects (the active, or agent, intellect). The agent intellect al-Kindi considered to be a spiritual being or substance distinct from the human soul and outside the individual person.
Following the precedent of Alexander of Aphrodisias, al-Kindi thus identified the agent intellect of Aristotle with the last of the Neoplatonic Intelligences that emanate from God to effect and sustain creation. This concept of one separate agent intellect for all men remained a major tenet of the Arabian philosophers. It explained human knowledge as a product of outside stimulation and, inasmuch as the human personality and soul were strongly dependent upon the active reason, it implied a denial of personal survival after death.
The Theory of the Magical Arts, the second most important work of al-Kindi, has even stronger Neoplatonic elements. It describes a celestial harmony based on an emanation of light and being from God. Each part of the universe reflects the order of the whole.
It was possibly in the context of the Neoplatonic interest in light rays that al-Kindi explored the field of optics. In a work translated as De aspectibus, he discussed the passage of light in a straight line and the effect of a looking glass on the process of vision. In a treatise on sky color, he discussed the effect of dust and vapor.
The works of al-Kindi set the foundation for the achievements of Arabian philosophy and science. Later scholars adopted his belief that mathematics was the basis of science. The type of questions raised and the explanation of Aristotelian concepts in Neoplatonic terms established a pattern for later Arabian philosophers. Although not the most famous thinker in Islamic philosophy, al-Kindi began a movement of great importance in both European and Islamic civilizations.
There is, at present, no major work in English on al-Kindi. Discussions of his philosophical contribution can be found in Tjitze J. de Boer, The History of Philosophy in Islam (1901; trans. 1903), and Julius Weinberg, A Short History of Medieval Philosophy (1964). The scientific contribution of al-Kindi is best described in George Sarton, Introduction to the History of Science, vol. 1 (1927).