# Hunayn ibn Ishaq

Hunayn ibn Ishaq   known in Latin as Johannitius) (809–873) was a famous and influential Assyrian[1] Nestorian Christian scholar, physician, and scientist, known for his work in translating Greek scientific and medical works into Arabic and Syriac during the heyday of the Islamic Abbasid Caliphate. Ḥunayn ibn Isḥaq was the most productive translator of Greek medical and scientific treatises in his day. He studied Greek and became known among the Arabs as the “Sheikh of the translators.” He mastered four languages: Arabic, Syriac, Greek and Persian. His translations did not require corrections. Hunayn’s method was widely followed by later translators. He was originally from southern Iraq but he spent his working life in Baghdad, the center of the great ninth-century Greek-into-Arabic/Syriac translation movement. His fame went far beyond his own community

## Overview

In the Abbasid era, a new interest in extending the study of Greek science had arisen. At that time, there was a vast amount of untranslated ancient Greek literature pertaining to philosophy, mathematics, natural science, and medicine.[3][4] This valuable information was only accessible to a very small minority of Middle Eastern scholars who knew the Greek language; the need for an organized translation movement was urgent. In time, Hunayn ibn Ishaq became arguably the chief translator of the era, and laid the foundations of Islamic medicine.[3] In his lifetime, ibn Ishaq translated 116 works, including Plato’s Timaeus, Aristotle’s Metaphysics, and the Old Testament, into Syriac and Arabic.[4][5] Ibn Ishaq also produced 36 of his own books, 21 of which covered the field of medicine.[5] His son Ishaq, and his nephew Hubaysh, worked together with him at times to help translate. Hunayn ibn Ishaq is known for his translations, his method of translation, and his contributions to medicine.[4]

## Early life

Hunayn ibn Ishaq was an Assyrian Christian born in 809, during the Abbasid period, in al-Hira, Iraq.[6][7] As a child, he learned the Syriac and Arabic languages. Although al-Hira was known for commerce and banking, and his father was a pharmacist, Hunayn went to Baghdad in order to study medicine. In Baghdad, Hunayn had the privilege to study under renowned physician Yuhanna ibn Masawayh; however, Hunayn’s countless questions irritated Yuhanna, causing him to scold Hunayn and forcing him to leave. Hunayn promised himself to return to Baghdad when he became a physician. He went abroad to master the Latin language. On his return to Baghdad, Hunayn displayed his newly acquired skills by reciting the works of Homer and Galen. In awe, ibn Masawayh reconciled with Hunayn, and the two started to work cooperatively.[7]

Hunayn was extremely motivated in his work to master Greek studies, which enabled him to translate Greek texts into Syriac and Arabic. The Abbasid Caliph al-Mamun noticed Hunayn’s talents and placed him in charge of the House of Wisdom, “Bayt al Hikmah.” The House of Wisdom was an institution where Greek works were translated and made available to scholars.[6] (Silvain Gougenheim argued, though, that there is no evidence of Hunayn being in charge of “Bayt al Hikham”[8]) The caliph also gave Hunayn the opportunity to travel to Byzantium in search of additional manuscripts, such as those of Aristotle and other prominent authors.[7]

## Accomplishments

In Hunayn ibn Ishaq’s lifetime, he devoted himself to working on a multitude of writings; both translations and original works.[7]

### As a writer of original work

Hunayn wrote on a variety of subjects that included philosophy, religion and medicine. In “How to Grasp Religion,” Hunayn explains the truths of religion that include miracles not possibly made by humans and humans’ incapacity to explain facts about some phenomena, and false notions of religion that include depression and an inclination for glory. He worked on Arabic grammar and lexicography.[7]

#### Ophthalmology

Hunayn ibn Ishaq enriched the field of ophthalmology. His developments in the study of the human eye can be traced through his innovative book, “Ten Treatises on Ophthalmology.” This textbook is the first known systematic treatment of this field and was most likely used in medical schools at the time. Throughout the book, Hunayn explains the eye and its anatomy in minute detail; its diseases, their symptoms, their treatments. He discusses the nature of cysts and tumors, and the swelling they cause. He discusses how to treat various corneal ulcers through surgery, and the therapy involved in repairing cataracts. “Ten Treatises on Ophthalmology” demonstrates the skills Hunayn ibn Ishaq had not just as a translator and a physician, but also as a surgeon.[5]

### As a physician

Hunayn ibn Ishaq’s reputation as a scholar and translator, and his close relationship with Caliph al-Mutawakkil, led the caliph to name Hunayn as his personal physician, ending the exclusive use of physicians from the Bukhtishu family.[7] Despite their relationship, the caliph became distrustful; at the time, there were fears of death from poisoning, and physicians were well aware of its synthesis procedure. The caliph tested Hunayn’s ethics as a physician by asking him to formulate a poison, to be used against a foe, in exchange for a large sum. Hunayn ibn Ishaq repeatedly rejected the Caliph’s generous offers, saying he would need time to develop a poison. Disappointed, the caliph imprisoned his physician for a year. When asked why he would rather be killed than make the drug, Hunayn explained the physician’s oath required him to help, and not harm, his patients.[6]

### As a translator

Some of Hunayn’s most notable translations were his translation of “De materia Medica,” which was technically a pharmaceutical handbook, and his most popular selection, “Questions on Medicine.”[4] “Questions on Medicine” was extremely beneficial to medical students because it was a good guide for beginners to become familiar with the fundamental aspects of medicine in order to understand the more difficult materials. Information was presented in the form of question and answer. The questions were taken from Galen’s “Art of Physic,” and the answers were based on “Summaria Alexandrinorum.” For instance, Hunayn answers what the four elements and four humors are and also explains that medicine is divided into therapy and practice. He goes on later to define health, disease, neutrality, and also natural and contranatural, which associates with the six necessary causes to live healthy.[7]

Hunayn translated writings on agriculture, stones, and religion. He translated some of Plato’s and Aristotle’s works, and the commentaries of ancient Greeks. Additionally, Hunayn translated many medicinal texts and summaries, mainly those of Galen. He translated a countless number of Galen’s works including “On Sects” and “On Anatomy of the Veins and Arteries.”[7]Many published works of R. Duval in Chemistry represent translations of Hunayn’s work.[9] Also in Chemistry a book titled [‘An Al-Asma’] meaning “About the Names”, did not reach researchers but was used in “Dictionary of Ibn Bahlool” of the 10th century.

#### Translation techniques

In his efforts to translate as much Greek material as possible, Hunayn ibn Ishaq was accompanied by his son Ishaq ibn Hunayn and his nephew Hubaysh. It was quite normal at times for Hunayn to translate Greek material into Syriac, and have his nephew finish by translating the text from Syriac to Arabic. Ishaq corrected his partners’ errors while translating writings in Greek and Syriac into Arabic.[4]

Unlike other translators in the Abbasid period, Hunayn opposed translating texts word for word. Instead, he would attempt to attain the meaning of the subject and the sentences, and then in a new manuscript, rewrite the piece of knowledge in Syriac or Arabic.[4] He also corrected texts by collecting different set of books revolving around a subject and by finalizing the meaning of the subject.[7] The method helped gather, in just 100 years, nearly all the knowledge from Greek medicine.[4].[2]

# Alhazen

Abū ʿAlī al-Ḥasan ibn al-Ḥasan ibn al-Haytham  (965 in Basrac. 1040 in Cairo) was a Muslim[5] scientist, polymath, mathematician, astronomer and philosopher, described in various sources as either an Arab or Persian.[1][6] He made significant contributions to the principles of optics, as well as to astronomy, mathematics, visual perception, and to the scientific method. He also wrote insightful commentaries on works by Aristotle, Ptolemy, and the Greek mathematician Euclid.[7]He is frequently referred to as Ibn al-Haytham, and sometimes as al-Basri (Arabic: البصري), after his birthplace in the city of Basra.[8] He was also nicknamed Ptolemaeus Secundus (“Ptolemy the Second”)[9] or simply “The Physicist”[10] in medieval Europe.

Born circa 965, in Basra, present-day Iraq, he lived mainly in Cairo, Egypt, dying there at age 74.[9] According to one version of his biography, overconfident about practical application of his mathematical knowledge, he assumed that he could regulate the floods of the Nile.[11] After being ordered by Al-Hakim bi-Amr Allah, the sixth ruler of the Fatimid caliphate, to carry out this operation, he quickly perceived the impossibility of what he was attempting to do. Fearing for his life, he feigned madness[1][12] and was placed under house arrest, during which he undertook scientific work. After the death of Al-Hakim he was able to prove that he was not mad, and for the rest of his life he made money copying texts while writing mathematical works and teaching.[13] He is known as the “Father of Modern Optics, Experimental physics and Scientific methodology[14][15][16][17] and could be regarded as the first theoretical physicist

### Biography

Alhazen was born in Basra, in the Iraq province of the Buyid Empire.[1] He probably died in Cairo, Egypt. During the Islamic Golden Age, Basra was a “key beginning of learning”,[18] and he was educated there and in Baghdad, the capital of the Abbasid Caliphate, and the focus of the “high point of Islamic civilization”.[18] During his time in Buyid Iran, he worked as what could be described as a civil servant and studied maths and science.[8][19]One account of his career has him called to Egypt by Al-Hakim bi-Amr Allah, ruler of the Fatimid Caliphate, to regulate the flooding of the Nile, a task requiring an early attempt at building a dam at the present site of the Aswan Dam.[20] After deciding the scheme was impractical and fearing the caliph’s anger, he feigned madness.

He was kept under house arrest from 1011 until al-Hakim’s death in 1021.[21] During this time, he wrote his influential Book of Optics. After his house arrest ended, he wrote scores of other treatises on physics, astronomy and mathematics. He later traveled to Islamic Spain. During this period, he had ample time for his scientific pursuits, which included optics, mathematics, physics, medicine, and practical experiments. Some biographers have claimed that Alhazen fled to Syria, ventured into Baghdad later in his life, or was in Basra when he pretended to be insane. In any case, he was in Egypt by 1038.[8] During his time in Cairo, he contributed to the work of Dar-el-Hikma, the city’s “House of Wisdom”.[22]Among his students were Sorkhab (Sohrab), a Persian student who was one of the greatest people of Iran‘s Semnan and was his student for over 3 years, and Abu al-Wafa Mubashir ibn Fatek, an Egyptian scientist who learned mathematics from Alhazan.[23]

### Legacy

Alhazen made significant improvements in optics, physical science, and the scientific method. Alhazen’s work on optics is credited with contributing a new emphasis on experiment. The Latin translation of his main work, Kitab al-Manazir (Book of Optics),[24] exerted a great influence on Western science: for example, on the work of Roger Bacon, who cites him by name.[25] His research in catoptrics (the study of optical systems using mirrors) centred on spherical and parabolic mirrors and spherical aberration. He made the observation that the ratio between the angle of incidence and refraction does not remain constant, and investigated the magnifying power of a lens. His work on catoptrics also contains the problem known as “Alhazen’s problem“.[26]

Meanwhile in the Islamic world, Alhazen’s work influenced Averroes‘ writings on optics,[27] and his legacy was further advanced through the ‘reforming’ of his Optics by Persian scientist Kamal al-Din al-Farisi (d. ca. 1320) in the latter’s Kitab Tanqih al-Manazir (The Revision of [Ibn al-Haytham’s] Optics).[28] He wrote as many as 200 books, although only 55 have survived, and many of those have not yet been translated from Arabic.[citation needed] Some of his treatises on optics survived only through Latin translation. During the Middle Ages his books on cosmology were translated into Latin, Hebrew and other languages. The crater Alhazen on the Moon is named in his honour,[29] as was the asteroid 59239 Alhazen.[30] In honour of Alhazen, the Aga Khan University (Pakistan) named its Ophthalmology endowed chair as “The Ibn-e-Haitham Associate Professor and Chief of Ophthalmology”.[31] Alhazen (by the name Ibn al-Haytham) is featured on the obverse of the Iraqi 10,000 dinars banknote issued in 2003,[32] and on 10 dinar notes from 1982. A research facility that UN weapons inspectors suspected of conducting chemical and biological weapons research in Saddam Hussein’s Iraq was also named after him.[32][33]

## Book of Optics

Alhazen’s most famous work is his seven volume treatise on optics, Kitab al-Manazir (Book of Optics), written from 1011 to 1021. Optics was translated into Latin by an unknown scholar at the end of the 12th century or the beginning of the 13th century.[34] It was printed by Friedrich Risner in 1572, with the title Opticae thesaurus: Alhazeni Arabis libri septem, nuncprimum editi; Eiusdem liber De Crepusculis et nubium ascensionibus (English : Optics treasure: Arab Alhazeni seven books, published for the first time: The book of the Twilight of the clouds and ascensions).[35] Risner is also the author of the name variant “Alhazen”; before Risner he was known in the west as Alhacen, which is the correct transcription of the Arabic name.[36] This work enjoyed a great reputation during the Middle Ages. Works by Alhazen on geometric subjects were discovered in the Bibliothèque nationale in Paris in 1834 by E. A. Sedillot. Other manuscripts are preserved in the Bodleian Library at Oxford and in the library of Leiden.

### Theory of Vision

Two major theories on vision prevailed in classical antiquity. The first theory, the emission theory, was supported by such thinkers as Euclid and Ptolemy, who believed that sight worked by the eye emitting rays of light. The second theory, the intromission theory supported by Aristotle and his followers, had physical forms entering the eye from an object. Previous Islamic writers (such as al-Kindi) had argued essentially on Euclidean, Galenist, or Aristotelian lines; Alhazen’s achievement was to come up with a theory which successfully combined parts of the mathematical ray arguments of Euclid, the medical tradition of Galen, and the intromission theories of Aristotle. Alhazen’s intromission theory followed al-Kindi (and broke with Aristotle) in asserting that “from each point of every colored body, illuminated by any light, issue light and color along every straight line that can be drawn from that point”.[37]

This however left him with the problem of explaining how a coherent image was formed from many independent sources of radiation; in particular, every point of an object would send rays to every point on the eye. What Alhazen needed was for each point on an object to correspond to one point only on the eye.[37] He attempted to resolve this by asserting that only perpendicular rays from the object would be perceived by the eye; for any one point on the eye, only the ray which reached it directly, without being refracted by any other part of the eye, would be perceived. He argued using a physical analogy that perpendicular rays were stronger than oblique rays; in the same way that a ball thrown directly at a board might break the board, whereas a ball thrown obliquely at the board would glance off, perpendicular rays were stronger than refracted rays, and it was only perpendicular rays which were perceived by the eye. As there was only one perpendicular ray that would enter the eye at any one point, and all these rays would converge on the centre of the eye in a cone, this allowed him to resolve the problem of each point on an object sending many rays to the eye; if only the perpendicular ray mattered, then he had a one-to-one correspondence and the confusion could be resolved.[38] He later asserted (in book seven of the Optics) that other rays would be refracted through the eye and perceived as if perpendicular.[39]

His arguments regarding perpendicular rays do not clearly explain why only perpendicular rays were perceived; why would the weaker oblique rays not be perceived more weakly?[40] His later argument that refracted rays would be perceived as if perpendicular does not seem persuasive.[41] However, despite its weaknesses, no other theory of the time was so comprehensive, and it was enormously influential, particularly in Western Europe: “Directly or indirectly, his De Aspectibus inspired much of the activity in optics which occurred between the 13th and 17th centuries.” [42] Kepler‘s later theory of the retinal image (which resolved the problem of the correspondence of points on an object and points in the eye) built directly on the conceptual framework of Alhazen.[42]Alhazen showed through experiment that light travels in straight lines, and carried out various experiments with lenses, mirrors, refraction, and reflection.[26] He was the first to consider separately the vertical and horizontal components of reflected and refracted light rays, which was an important step in understanding optics geometrically.[43]

The camera obscura was known to the Chinese, and Aristotle had discussed the principle behind it in his Problems, however it is Alhazen’s work which contains the first clear description[44] and early analysis[45] of the device. Alhazen studied the process of sight, the structure of the eye, image formation in the eye, and the visual system. Ian P. Howard argued in a 1996 Perception article that Alhazen should be credited with many discoveries and theories which were previously attributed to Western Europeans writing centuries later. For example, he described what became in the 19th century Hering’s law of equal innervation; he had a description of vertical horopters which predates Aguilonius by 600 years and is actually closer to the modern definition than Aguilonius’s; and his work on binocular disparity was repeated by Panum in 1858.[46]

Craig Aaen-Stockdale, while agreeing that Alhazen should be credited with many advances, has expressed some caution, especially when considering Alhazen in isolation from Ptolemy, who Alhazen was extremely familiar with. Alhazen corrected a significant error of Ptolemy regarding binocular vision, but otherwise his account is very similar; Ptolemy also attempted to explain what is now called Hering’s law.[47] In general, Alhazen built on and expanded the optics of Ptolemy.[48][49] In a more detailed account of Ibn al-Haytham’s contribution to the study of binocular vision based on Lejeune[50] and Sabra,[11] Raynaud[51] showed that the concepts of correspondence, homonymous and crossed diplopia were in place in Ibn al-Haytham’s optics. But contrary to Howard, he explained why Ibn al-Haytham did not give the circular figure of the horopter and why, by reasoning experimentally, he was in fact closer to the discovery of Panum’s fusional area than that of the Vieth-Müller circle. In this regard, Ibn al-Haytham’s theory of binocular vision faced two main limits: the lack of recognition of the role of the retina, and obviously the lack of an experimental investigation of ocular tracts.

Alhazen’s most original contribution was that after describing how he thought the eye was anatomically constructed, he went on to consider how this anatomy would behave functionally as an optical system.[52] His understanding of pinhole projection from his experiments appears to have influenced his consideration of image inversion in the eye,[53] which he sought to avoid.[54] He maintained that the rays that fell perpendicularly on the lens (or glacial humor as he called it) were further refracted outward as they left the glacial humor and the resulting image thus passed upright into the optic nerve at the back of the eye.[55] He followed Galen in believing that the lens was the receptive organ of sight, although some of his work hints that he thought the retina was also involved.[56].[15]

# Al-Kindi

Abu Yūsuf Yaʻqūb ibn ʼIsḥāq aṣ-Ṣabbāḥ al-Kindī   (c. 801–873 CE), known as “the Philosopher of the Arabs”, was an Iraqi Muslim Arab philosopher, mathematician, physician, and musician. Al-Kindi was the first of the Muslim peripatetic philosophers, and is unanimously hailed as the “father of Islamic or Arabic philosophy[2][3][4] for his synthesis, adaptation and promotion of Greek and Hellenistic philosophy in the Muslim world.[5]Al-Kindi was a descendant of the Kinda tribe. He was born and educated in Basra,[6] before going to pursue further studies in Baghdad. Al-Kindi 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” (as Greek philosophy was often referred to by Muslim scholars) had a profound effect on his intellectual development, and led him to write hundreds of original treatises of his own on a range of subjects ranging from metaphysics, ethics, logic and psychology, to medicine, pharmacology,[7] mathematics, astronomy, astrology and optics, and further afield to more practical topics like perfumes, swords, jewels, glass, dyes, zoology, tides, mirrors, meteorology and earthquakes.[8][9]In the field of mathematics, al-Kindi played an important role in introducing Indian numerals to the Islamic and Christian world.[10] He was a pioneer in cryptanalysis and devised several new methods of breaking ciphers.[11

Using his mathematical and medical expertise, he was able to develop a scale that would allow doctors to quantify the potency of their medication.[12]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.[13] But despite the important role he played in making philosophy accessible to Muslim intellectuals, his own philosophical output was largely overshadowed by that of al-Farabi and very few of his texts are available for modern scholars to examine.

## Life

Al-Kindi was born in Kufa to an aristocratic family of the Kinda tribe. His father was the governor of Kufa, 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 House of Wisdom, a recently established centre 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 al-Mutawakkil.[14]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.[14]After his death, al-Kindi’s philosophical works quickly fell into obscurity and many of them were lost even to later Islamic scholars and historians. Felix Klein-Franke suggests a number of reasons for this: aside from the militant orthodoxy of al-Mutawakkil, the Mongols also destroyed countless libraries during their invasion. 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.[15]

## Accomplishments

Al-Kindi was a master of many different areas of thought. And although he would eventually be eclipsed by names such as al-Farabi and Avicenna, he was held to be one of the greatest Islamic philosophers of his time. The Italian Renaissance scholar Geralomo Cardano (1501–1575) considered him one of the twelve greatest minds of the Middle Ages.[16] According to 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).[17] His influence in the fields of physics, mathematics, medicine, philosophy and music were far-reaching and lasted for several centuries. 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.[18]

### 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 (Bayt al-Hikma), an institute of translation and learning patronized by the Abbasid Caliphs, in Baghdad.[14] 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.[19]

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.[20]

### 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.[21]

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 his treatise On Rays is that the planets exercise their influence in straight lines. In each of these, he presents two fundamentally different views of physical interaction; action by contact and action at a distance. This dichotomy is duplicated in his writings on optics.[22]
Some of the notable astrological works by al-Kindi include:[23]
• 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, (“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

Two major theories of optics appear in the writings of al-Kindi; Aristotelian and Euclidian. 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, Euclidian 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 (something which is commonly accepted in modern science). For this reason, al-Kindi considered the latter preponderant.[24]Through the Latin version of the De Aspectibus, Al-Kindi partly influenced the optical investigations of Robert Grosseteste.[25]

### 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.[26] 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 drug and a system, based the phases of the moon, that would allow a doctor to determine in advance the most critical days of a patient’s illness.[12]

### Chemistry

As an advanced chemist, he was also an opponent of alchemy; he debunked the myth that simple, base metals could be transformed into precious metals such as gold or silver.[27] He is sometimes credited as one of the first distillers of alcohol.

### Mathematics

Al-Kindi authored works on a number of important mathematical subjects, including arithmetic, geometry, the Indian numbers, the harmony of numbers, lines and multiplication with numbers, relative quantities, measuring proportion and time, and numerical procedures and cancellation.[10] He also wrote four volumes, On the Use of the Indian Numerals (Ketab fi Isti’mal al-‘Adad al-Hindi) which contributed greatly to diffusion of the Indian system of numeration in the Middle-East and the West. In geometry, among other works, he wrote on the theory of parallels. Also related to geometry were two works on optics. One of the ways in which he made use of mathematics as a philosopher was to attempt to disprove the eternity of the world by demonstrating that actual infinity is a mathematical and logical absurdity.[28]

Ahmad ibn Muhammad al-Nahavandi was a Persian astronomer of the 8th and 9th centuries. His name indicates that he was from Nahavand, a city in Iran. He lived and worked at the Academy of Gundishapur, in Khuzestan, Iran, at the time of Yahya ibn Khalid ibn Barmak, who died in 803AD, where he is reported to have been making astronomical observations around the year 800AD. He and Mashallah ibn Athari were among the earliest Islamic era astronomers who flourished during the reign of al-Mansur, the second Abbasid Caliph. He also compiled tables called the comprehensive (Mushtamil).

Abū ʿAbdallāh Muḥammad ibn Mūsā al-Khwārizmī earlier transliterated as Algoritmi or   was a Persian[2][5] mathematician, astronomer and geographer during the Abbasid Empire, a scholar in the House of Wisdom in Baghdad. In the twelfth century, Latin translations of his work on the Indian numerals introduced the decimal positional number system to the Western world.[4] His Compendious Book on Calculation by Completion and Balancing presented the first systematic solution of linear and quadratic equations in Arabic. In Renaissance Europe, he was considered the original inventor of algebra, although it is now known that his work is based on older Indian or Greek sources.[6] He revised Ptolemy‘s Geography and wrote on astronomy and astrology. Some words reflect the importance of al-Khwarizmi’s contributions to mathematics. “Algebra” is derived from al-jabr, one of the two operations he used to solve quadratic equations. Algorism and algorithm stem from Algoritmi, the Latin form of his name.[7] His name is

## Life

He was born in a Persian[2][5] family, and his birthplace is given as Chorasmia[9] by Ibn al-Nadim.
Few details of al-Khwārizmī’s life are known with certainty. His name may indicate that he came from Khwarezm (Khiva), then in Greater Khorasan, which occupied the eastern part of the Greater Iran, now Xorazm Province in Uzbekistan. Al-Tabari gave his name as Muhammad ibn Musa al-Khwārizmī al-Majousi al-Katarbali (محمد بن موسى الخوارزميّ المجوسـيّ القطربّـليّ). The epithet al-Qutrubbulli could indicate he might instead have come from Qutrubbul (Qatrabbul),[10] a viticulture district near Baghdad. However, Rashed[11] suggests:
There is no need to be an expert on the period or a philologist to see that al-Tabari’s second citation should read “Muhammad ibn Mūsa al-Khwārizmī and al-Majūsi al-Qutrubbulli,” and that there are two people (al-Khwārizmī and al-Majūsi al-Qutrubbulli) between whom the letter wa [Arabic ‘و‘ for the article ‘and‘] has been omitted in an early copy. This would not be worth mentioning if a series of errors concerning the personality of al-Khwārizmī, occasionally even the origins of his knowledge, had not been made. Recently, G. J. Toomer … with naive confidence constructed an entire fantasy on the error which cannot be denied the merit of amusing the reader.
Regarding al-Khwārizmī’s religion, Toomer writes:
Another epithet given to him by al-Ṭabarī, “al-Majūsī,” would seem to indicate that he was an adherent of the old Zoroastrian religion. This would still have been possible at that time for a man of Iranian origin, but the pious preface to al-Khwārizmī’s Algebra shows that he was an orthodox Muslim, so al-Ṭabarī’s epithet could mean no more than that his forebears, and perhaps he in his youth, had been Zoroastrians.[12]
Ibn al-Nadīm‘s Kitāb al-Fihrist includes a short biography on al-Khwārizmī, together with a list of the books he wrote. Al-Khwārizmī accomplished most of his work in the period between 813 and 833. After the Islamic conquest of Persia, Baghdad became the centre of scientific studies and trade, and many merchants and scientists from as far as China and India traveled to this city, as did Al-Khwārizmī. He worked in Baghdad as a scholar at the House of Wisdom established by Caliph al-Maʾmūn, where he studied the sciences and mathematics, which included the translation of Greek and Sanskrit scientific manuscripts. D. M. Dunlop suggests that it may have been possible that Muḥammad ibn Mūsā al-Khwārizmī was in fact the same person as Muḥammad ibn Mūsā ibn Shākir, the eldest of the three Banū Mūsā.[13][year missing]

## Contributions

Al-Khwārizmī’s contributions to mathematics, geography, astronomy, and cartography established the basis for innovation in algebra and trigonometry. His systematic approach to solving linear and quadratic equations led to algebra, a word derived from the title of his 830 book on the subject, “The Compendious Book on Calculation by Completion and Balancing” (al-Kitab al-mukhtasar fi hisab al-jabr wa’l-muqabalaالكتاب المختصر في حساب الجبر والمقابلة). On the Calculation with Hindu Numerals written about 825, was principally responsible for spreading the Indian system of numeration throughout the Middle East and Europe. It was translated into Latin as Algoritmi de numero Indorum. Al-Khwārizmī, rendered as (Latin) Algoritmi, led to the term “algorithm“. Some of his work was based on Persian and Babylonian astronomy, Indian numbers, and Greek mathematics.

Al-Khwārizmī systematized and corrected Ptolemy‘s data for Africa and the Middle East. Another major book was Kitab surat al-ard (“The Image of the Earth”; translated as Geography), presenting the coordinates of places based on those in the Geography of Ptolemy but with improved values for the Mediterranean Sea, Asia, and Africa. He also wrote on mechanical devices like the astrolabe and sundial. He assisted a project to determine the circumference of the Earth and in making a world map for al-Ma’mun, the caliph, overseeing 70 geographers.[14]When, in the 12th century, his works spread to Europe through Latin translations, it had a profound impact on the advance of mathematics in Europe. He introduced Arabic numerals into the Latin West, based on a place-value decimal system developed from Indian sources.[15]

### Algebra

Al-Kitāb al-mukhtaṣar fī ḥisāb al-jabr wa-l-muqābala (Arabic: الكتاب المختصر في حساب الجبر والمقابلة‎, ‘The Compendious Book on Calculation by Completion and Balancing’) is a mathematical book written approximately 830 CE. The book was written with the encouragement of the Caliph al-Ma’mun as a popular work on calculation and is replete with examples and applications to a wide range of problems in trade, surveying and legal inheritance.[16] The term algebra is derived from the name of one of the basic operations with equations (al-jabr, meaning completion, or, subtracting a number from both sides of the equation) described in this book. The book was translated in Latin as Liber algebrae et almucabala by Robert of Chester (Segovia, 1145) hence “algebra”, and also by Gerard of Cremona. A unique Arabic copy is kept at Oxford and was translated in 1831 by F. Rosen. A Latin translation is kept in Cambridge.[17]
It provided an exhaustive account of solving polynomial equations up to the second degree,[18] and discussed the fundamental methods of “reduction” and “balancing”, referring to the transposition of subtracted terms to the other side of an equation, that is, the cancellation of like terms on opposite sides of the equation.[19]
Al-Khwārizmī’s method of solving linear and quadratic equations worked by first reducing the equation to one of six standard forms (where b and c are positive integers)
• squares equal roots (ax2 = bx)
• squares equal number (ax2 = c)
• roots equal number (bx = c)
• squares and roots equal number (ax2 + bx = c)
• squares and number equal roots (ax2 + c = bx)
• roots and number equal squares (bx + c = ax2)
by dividing out the coefficient of the square and using the two operations al-jabr (Arabic: الجبر‎ “restoring” or “completion”) and al-muqābala (“balancing”). Al-jabr is the process of removing negative units, roots and squares from the equation by adding the same quantity to each side. For example, x2 = 40x − 4x2 is reduced to 5x2 = 40x. Al-muqābala is the process of bringing quantities of the same type to the same side of the equation. For example, x2 + 14 = x + 5 is reduced to x2 + 9 = x.
The above discussion uses modern mathematical notation for the types of problems which the book discusses. However, in al-Khwārizmī’s day, most of this notation had not yet been invented, so he had to use ordinary text to present problems and their solutions. For example, for one problem he writes, (from an 1831 translation)
“If some one say: “You divide ten into two parts: multiply the one by itself; it will be equal to the other taken eighty-one times.” Computation: You say, ten less thing, multiplied by itself, is a hundred plus a square less twenty things, and this is equal to eighty-one things. Separate the twenty things from a hundred and a square, and add them to eighty-one. It will then be a hundred plus a square, which is equal to a hundred and one roots. Halve the roots; the moiety is fifty and a half. Multiply this by itself, it is two thousand five hundred and fifty and a quarter. Subtract from this one hundred; the remainder is two thousand four hundred and fifty and a quarter. Extract the root from this; it is forty-nine and a half. Subtract this from the moiety of the roots, which is fifty and a half. There remains one, and this is one of the two parts.”[16]
In modern notation this process, with ‘x’ the “thing” (shay’) or “root”, is given by the steps,
$(10-x)^2=81 x$
$x^2 - 20 x + 100 = 81 x$
$x^2+100=101 x$
Let the roots of the equation be ‘p’ and ‘q’. Then $\tfrac{p+q}{2}=50\tfrac{1}{2}$, $pq =100$ and
$\frac{p-q}{2} = \sqrt{\left(\frac{p+q}{2}\right)^2 - pq}=\sqrt{2550\tfrac{1}{4} - 100}=49\tfrac{1}{2}$
So a root is given by
$x=50\tfrac{1}{2}-49\tfrac{1}{2}=1$
Several authors have also published texts under the name of Kitāb al-jabr wa-l-muqābala, including |Abū Ḥanīfa al-Dīnawarī, Abū Kāmil Shujā ibn Aslam, Abū Muḥammad al-ʿAdlī, Abū Yūsuf al-Miṣṣīṣī, ‘Abd al-Hamīd ibn Turk, Sind ibn ʿAlī, Sahl ibn Bišr, and Šarafaddīn al-Ṭūsī.
J. J. O’Conner and E. F. Robertson wrote in the MacTutor History of Mathematics archive:
“Perhaps one of the most significant advances made by Arabic mathematics began at this time with the work of al-Khwarizmi, namely the beginnings of algebra. It is important to understand just how significant this new idea was. It was a revolutionary move away from the Greek concept of mathematics which was essentially geometry. Algebra was a unifying theory which allowed rational numbers, irrational numbers, geometrical magnitudes, etc., to all be treated as “algebraic objects”. It gave mathematics a whole new development path so much broader in concept to that which had existed before, and provided a vehicle for future development of the subject. Another important aspect of the introduction of algebraic ideas was that it allowed mathematics to be applied to itself in a way which had not happened before.”[20]
R. Rashed and Angela Armstrong write:
“Al-Khwarizmi’s text can be seen to be distinct not only from the Babylonian tablets, but also from DiophantusArithmetica. It no longer concerns a series of problems to be resolved, but an exposition which starts with primitive terms in which the combinations must give all possible prototypes for equations, which henceforward explicitly constitute the true object of study. On the other hand, the idea of an equation for its own sake appears from the beginning and, one could say, in a generic manner, insofar as it does not simply emerge in the course of solving a problem, but is specifically called on to define an infinite class of problems.”[21]also the origin of (Spanish) guarismo[8] and of (Portuguese) algarismo, both meaning digit.