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Abū Yūsuf Yaʻqūb ibn ʼIsḥāq aṣ-Ṣabbāḥ al-Kindī (; ; ; ) was an Arab Muslim polymath 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 Ancient Greece scientific and philosophical texts into the Arabic language. This contact with "the philosophy of the ancients" (as Hellenistic philosophy was often referred to by Muslim scholars) 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 (717–786), 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 revelation.
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 (as well as of non-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 Mongol Empire 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.
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 kalam on the other (though in fact he rejected speculative theology). 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 (mainly because he was the first philosopher writing in the Arabic language), he successfully incorporated Aristotle and (especially) neo-Platonism thought into an Islamic philosophical framework. This was an important factor in the introduction and popularization of Greek philosophy in the Muslim intellectual world.
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 ( De radiis) 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:
Two major theories of optics appear in the writings of al-Kindi: Aristotle and . 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 (such as air) 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 Opus Majus.
One work attributed to al-Kindi, variously known as the Kitāb al-Taraffuq fī l-ʿiṭr ("The Book of Gentleness on Perfume") or the Kitāb Kīmiyāʾ al-ʿiṭr wa-l-taṣʿīdāt ("The Book of the Chemistry of Perfume and Distillations"), contains one of the earliest known references to the distillation of wine. (2025). 9789231038310, UNESCO. ISBN 9789231038310 pp. 65-69; (same content also available on the author's website). See also . The work also describes the distillation process for extracting , and provides recipes for 107 different kinds of perfumes.
One way to solve an encrypted message, if we know its language, is to find a different plaintext of the same language long enough to fill one sheet or so, and then we count the occurrences of each letter. We call the most frequently occurring letter the "first", the next most occurring letter the "second", the following most occurring letter the "third", and so on, until we account for all the different letters in the plaintext sample. Then we look at the cipher text we want to solve and we also classify its symbols. We find the most occurring symbol and change it to the form of the "first" letter of the plaintext sample, the next most common symbol is changed to the form of the "second" letter, and the following most common symbol is changed to the form of the "third" letter, and so on, until we account for all symbols of the cryptogram we want to solve.
Al-Kindi was influenced by the work of al-Khalil (717–786), who wrote the Book of Cryptographic Messages, which contains the first use of to list all possible Arabic language words with and without vowels.
One can also observe by the senses... how in consequence of extreme cold air changes into water. To do this, one takes a glass bottle, fills it completely with snow, and closes its end carefully. Then one determines its weight by weighing. One places it in a container... which has previously been weighed. On the surface of the bottle the air changes into water, and appears upon it like the drops on large porous pitchers, so that a considerable amount of water gradually collects inside the container. One then weighs the bottle, the water and the container, and finds their weight greater than previously, which proves the change. ... Some foolish persons are of opinion that the snow exudes through the glass. This is impossible. There is no process by which water or snow can be made to pass through glass.
In explaining the natural cause of the wind, and the difference for its directions based on time and location, he wrote:
When the sun is in its northern declination northerly places will heat up and it will be cold towards the south. Then the northern air will expand in a southerly direction because of the heat due to the contraction of the southern air. Therefore most of the summer winds are merits and most of the winter winds are not.
His works include discussions on the music therapy and what he regarded as "cosmological connections" of music.
Central to al-Kindi's understanding of metaphysics is God's tawhid, which he considers an attribute uniquely associated with God (and therefore not shared with anything else). By this he means that while we may think of any existent thing as being "one", it is in fact both "one" and many". For example, he says that while a body is one, it is also composed of many different parts. A person might say "I see an elephant", by which he means "I see one elephant", but the term 'elephant' refers to a species of animal that contains many. Therefore, only God is absolutely one, both in being and in concept, lacking any multiplicity whatsoever. Some feel this understanding entails a very rigorous negative theology because it implies that any description which can be predicated to anything else, cannot be said about God.
In addition to absolute oneness, al-Kindi also described God as the Creator. This means that He acts as both a final and efficient cause. Unlike later Muslim Neo-Platonism philosophers (who asserted that the universe existed as a result of God's existence "overflowing", which is a passive act), al-Kindi conceived of God as an active agent. In fact, of God as the agent, because all other intermediary agencies are contingent upon Him. The key idea here is that God "acts" through created intermediaries, which in turn "act" on one another – through a chain of cause and effect – to produce the desired result. In reality, these intermediary agents do not "act" at all, they are merely a conduit for God's own action. This is especially significant in the development of Islamic philosophy, as it portrayed the "first cause" and "unmoved mover" of Aristotelianism philosophy as compatible with the concept of God according to Islamic revelation.
According to Plato, everything that exists in the material world corresponds to certain universals in the heavenly realm. These forms are really abstract concepts such as a species, quality or relation, which apply to all physical objects and beings. For example, a red apple has the quality of "redness" derived from the appropriate universal. However, al-Kindi says that human intellects are only potentially able to comprehend these. This potential is actualized by the First Intellect, which is perpetually thinking about all of the universals. He argues that the external agency of this intellect is necessary by saying that human beings cannot arrive at a universal concept merely through perception. In other words, an intellect cannot understand the species of a thing simply by examining one or more of its instances. According to him, this will only yield an inferior "sensible form", and not the universal form which we desire. The universal form can only be attained through contemplation and actualization by the First Intellect.
The analogy he provides to explain his theory is that of wood and fire. Wood, he argues, is potentially hot (just as a human is potentially thinking about a universal), and therefore requires something else which is already hot (such as fire) to actualize this. This means that for the human intellect to think about something, the First Intellect must already be thinking about it. Therefore, he says that the First Intellect must always be thinking about everything. Once the human intellect comprehends a universal by this process, it becomes part of the individual's "acquired intellect" and can be thought about whenever he or she wishes.
In addition to this, al-Kindi adopted a naturalistic view of prophetic visions. He argued that, through the faculty of "imagination" as conceived of in Aristotelian philosophy, certain "pure" and well-prepared souls, were able to receive information about future events. Significantly, he does not attribute such visions or dreams to revelation from God, but instead explains that imagination enables human beings to receive the "form" of something without needing to perceive the physical entity to which it refers. Therefore, it would seem to imply that anyone who has purified themselves would be able to receive such visions. It is precisely this idea, amongst other naturalistic explanations of prophetic miracles that al-Ghazali attacks in his Incoherence of the Philosophers.
During his life, al-Kindi was fortunate enough to enjoy the patronage of the pro-Mutazilite Caliphs al-Ma'mun and al-Mu'tasim, which meant he could carry out his philosophical speculations with relative ease. In his own time, al-Kindi would be criticized for extolling the "intellect" as being the most immanent creation in proximity to God, which was commonly held to be the position of the angels.Black, p168 He also engaged in disputations with certain Mutazilites, whom he attacked for their belief in atoms, as not all Mutazilites accepted the belief of atomism.Black, p169 But the real role of al-Kindi in the conflict between philosophers and theologians would be to prepare the ground for debate. His works, says Deborah Black, contained all the seeds of future controversy that would be fully realized in al-Ghazali's Incoherence of the Philosophers.Black, p171
The best man of his time, unique in his knowledge of all the ancient sciences. He is called the Philosopher of the Arabs. His books deal with different sciences, such as logic, philosophy, geometry, arithmetic, astronomy, etc. We have connected him with the natural philosophers because of his prominence in Science.
Al-Kindi's major contribution was his establishment of philosophy in the Islamic world and his efforts in trying to harmonize the philosophical investigation along with the Islamic theology and creed. The philosophical texts which were translated under his supervision would become the standard texts in the Islamic world for centuries to come, even after his influence has been eclipsed by later Philosophers.
Al-Kindi was also an important figure in medieval Europe. Several of his books got translated into Latin influencing Western authors like Robert Grosseteste and Roger Bacon. The Italian Renaissance scholar Gerolamo Cardano (1501–1575) considered him one of the twelve greatest minds.
In 1986, the Royal Commission for Riyadh City inaugurated the Al Kindi Plaza in the Diplomatic Quarter district of Riyadh, Saudi Arabia.
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