According to
Arab bibliographer Ibn al-Nadim, al-Kindi wrote at least two hundred and sixty books, contributing heavily to
geometry (thirty-two books), medicine and philosophy (twenty-two books each),
logic (nine books), and
physics (twelve books). 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, 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 (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
Aristotelian and (especially)
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 (the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and the stars) 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 (most notably the Sun); 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 (
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: •
The Book of the Judgement of the Stars, including
The Forty Chapters, on questions and elections. •
On the Stellar Rays (spurious) • Several epistles on weather and meteorology, including
De mutatione temporum, ("On the Changing of the Weather"). • Treatise on the Judgement of Eclipses. • Treatise on the Dominion of the Arabs and its Duration (used to predict the end of Arab rule). •
The Choices of Days (on elections). •
On the Revolutions of the Years (on mundane astrology and natal revolutions). •
De Signis Astronomiae Applicitis as Mediciam 'On the Signs of Astronomy as applied to Medicine' •
Treatise on the Spirituality of the Planets.
Optics , Medieval manuscripts, MS R.15.17 (937). 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 (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
Roger Bacon.
Medicine There are more than thirty treatises attributed to al-Kindi in the field of medicine, in which he was chiefly influenced by the ideas of
Galen. His most important work in this field is probably
De Gradibus, in which he demonstrates the application of mathematics to medicine, particularly in the field of pharmacology. For example, he developed a mathematical scale to quantify the strength of a drug, and a system (based on the phases of the moon) that would allow a doctor to determine in advance the most critical days of a patient's illness. According to Plinio Prioreschi, this was the first attempt at serious quantification in medicine.
Chemistry Al-Kindi denied the possibility of transmuting base metals into precious metals such as gold and silver, a position that was later attacked by the Persian alchemist and physician
Abu Bakr al-Razi (). 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. The work also describes the
distillation process for extracting
rose oils, and provides recipes for 107 different kinds of perfumes.
Mathematics Al-Kindi authored works on a number of important mathematical subjects, including arithmetic, geometry, the Hindu numbers, the harmony of numbers, lines and multiplication with numbers, relative quantities, measuring proportion and time, and numerical procedures and cancellation.
Cryptography by
frequency analysis. Al-Kindi is credited with developing a method whereby variations in the frequency of the occurrence of letters could be analyzed and exploited to break ciphers (i.e. cryptanalysis by
frequency analysis). 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
permutations and combinations to list all possible
Arabic words with and without vowels.
Meteorology In a treatise entitled as
Risala fi l-Illa al-Failali l-Madd wa l-Fazr (
Treatise on the Efficient Cause of the Flow and Ebb), al-Kindi presents a theory on
tides which "depends on the changes which take place in bodies owing to the rise and fall of temperature." In order to support his argument, he gave a description of a scientific experiment as follows: 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.
Music theory Al-Kindi was the first theoretician of music in the Arab-Islamic world whose works have come down to us. He transferred the
Greek tonal system to the Arabic lute. Although he used tone letters (i.e. the Arabic alphabet), he also transferred the Greek tonal names to Arabic. He added a fifth string to the
'ud. He is known to have written treatises on
music theory. The known indexes list a varying number of writings attributed to him, but only four have survived and can be attributed to him with certainty: •
Stringed Instruments from One String to Ten Strings. •
Epistle on Informative Parts on Music. •
Epistle on Modes and Tones. •
Epistle on the Knowledge of the Composition of Melodies. His works include discussions on the
therapeutic value of music and what he regarded as "cosmological connections" of music. ==Philosophical thought==