Towards the end of the 10th century, Ibn al-Nadim, son of a Baghdad , bookseller and boon companion of Abbasid caliphs, compiled an annotated bibliography of books that had passed through his hands during the course of his long and active life. The sheer number of books that he lists, to say nothing of the range of their subject matter, is astonishing: Aristotle appears beside Sindbad the Sailor, Euclid beside the stories of Goha, Plato beside the poems of'Antar ibn Shad-dad.
The most striking feature of Ibn al-Nadim's catalog, however, is the number of books dealing with science. In a chapter entitled The Reason Why Books on Philosophy and Other Ancient Sciences Became Plentiful in This Country, Ibn al-Nadim relates a strange story of how Aristotle appeared in a dream to the Caliph al-Ma'mun and assured him that there was no conflict between reason and revelation. Thus reassured, al-Ma'mun set about obtaining the works of the Greek philosophers, the first step toward founding the famous House of Wisdom, a center for the translation of Greek scientific works into Arabic. Ibn al-Nadim told the story this way:
This dream was one of the most definite reasons for the output of books. Between al-Ma'mun and the Byzantine emperor there was correspondence ... so al-Ma'mun wrote to the Byzantine emperor asking his permission to obtain a selection of old scientific manuscripts, stored and treasured in the country of the Byzantines. After first refusing, he finally complied, and al-Ma'mun sent forth a number of scholars, among them al-Hajjaj ibn Matar, Ibn al-Batrik, Salman, the director of the House of Wisdom and many others. They selected books from those they found and brought them back to al-Ma'mun, who ordered them to prepare translations of them.
Though the House of Wisdom was founded in 830, Abbasid interest in Greek science had begun almost with the founding of the dynasty in 750 and by the time the House of Wisdom was launched, that interest had already been expressed in a number of important fields. The first Arabic translations of the medical works of Galen and Hippocrates, for example, were made by the official translator of the second Abbasid caliph, al-Mansur, builder of Baghdad. These sparked the interest in medicine so characteristic of Islam.
In 809, the Caliph Harun al-Rashid founded the first hospital in the Islamic World, and within a short time no major city in the empire was without one. The translator of these medical texts died in 800 - the year that Charlemagne was crowned Holy Roman Emperor. His son, Ibn al-Batrick, was among those scholars sent by al-Ma'mun to the Byzantine court in search of manuscripts.
But why should an Abbasid caliph, upholder of the Holy Law of Islam, dream of Aristotle, pagan philosopher to an alien race? Why did the Muslim community, engaged first in the great excitement of the conquests, and later in the difficult and absorbing task of administration, trouble with the science and philosophy of the Greeks, the lore of Persia and the mathematics of India?
The answers to these questions lie in the extraordinary cross-fertilization of once separate intellectual traditions that occurred as a result of the Muslim conquests of the seventh and early eighth centuries. These conquests united the ancient civilizations of the Middle East - to say nothing of North Africa andSpain - under a single rule for the first time since Alexander the Great, and Baghdad, from its foundation in 763, became a meeting place for Persians, Greeks, Indians, Copts, Berbers, Sogdians, Turks and even Chinese.
These people spoke many different languages, represented a great variety of cultures and an even wider variety of religions. Jews, Christians - of every possible variety - Manicheans, Hindus, Buddhists and even pagans jostled each other in the streets of the new capital. Yet the Abbasids, who tended to encourage talented men whatever their origin, absorbed them all and they, eager to contribute their talents helped to transform the empire.
The most single striking effect of the unification - of Anatolia, Iran, Syria, Iraq, Egypt, Palestine, North Africa and Spain - under Islamic rule was the opening of formerly closed frontiers - frontiers that had been closed politically, linguistically and intellectually since the death of Alexander the Great in the fourth century B.C. The Arabic word which we translate as "conquest" literally means "openings" - futuh — and this was indeed the effect of the Muslim conquests. For centuries the Byzantines had been at war with the Persians; now that major political and cultural frontier had fallen and students from the ancient university at Gondeshapur were able to meet colleagues from the philosophical schools of Alexandria in the streets of Baghdad and the effects were dramatic: no less than a scientific renaissance. It was rather as if Russia and America were to be united under the benevolent rule of a third party and able to freely exchange scientific information.
At first, contacts between scholars of such different backgrounds were limited - because of the lack of a common language. But by the time al-Ma'mun conceived the idea of the House of Wisdom, Arabic had already become the language of international scholarship as well as the language of Divine Revelation - and this was one of the most significant events in the history of ideas. Greek, long the language of philosophical and scientific inquiry, gave way to Arabic, and it was through the lens of Arabic that Western scholars, after the long half-light of the Dark Ages, first looked on the pages of Plato and Aristotle.
Another intellectual strand which was woven into the pattern of Islamic intellectual life during the early Abbasid period was that of Persia. The Abbasid movement had its origin in Khorasan, and particularly in the oasis of Marv (now in the Soviet Union), which had been the home of a medical school under the Sassanids. The Barmakid family, which supplied the Abbasid caliphs with their advisors and prime ministers from 750 until 803, was responsible for fostering translations from Pahlavi historical and scientific works into Arabic. It was through Pahlavi that the Arabs first came into contact with the learning of India, which had a long tradition of intellectual activity in the fields of astronomy, medicine and mathematics.
The Barmakids were also responsible for establishing the first paper mill in Baghdad.
Until the capture of Chinese paper makers by Muslim forces at the Battle of Talas in 751, precious books - such as the Koran -had been written on parchment, while papyrus was used for ephemeral government documents. Neither was very suitable, parchment because its price was prohibitive, papyrus because it decayed in the damper, colder climates outside its native home of Egypt. Paper, on the other hand, was the perfect writing material: cheap, long-lasting and attractive. This "invention" - for so it was - had an effect on education and scholarship as important as the invention of printing in the 15th century. Books were now within the reach of everyone, and soon schools were attached to most mosques, and libraries became common.
Unlike the Byzantines, with their suspicion of classical science and philosophy, the Muslims were actively enjoined by the Traditions - the dicta of the Prophet - to "seek learning, though it be in China." Another well-known Tradition states: "The search for knowledge is obligatory for every Muslim"; another that "The ink of scholars is worth more than the blood of martyrs."
In obedience to these injunctions, the first generations of Muslim scholars had devoted themselves to making the language of the Koran a vehicle for the expression of scientific ideas. Now, with the establishment of the House of Wisdom, with its library and staff of scholar-translators, the work could begin.
The job that lay before these men was Herculean. It was nothing less than the transfer of what had survived of the philosophical and scientific tradition of the ancient world - first into the Arabic language, and then into the conceptual framework of Islam. In the process, old errors were corrected and the experimental method, the basis of all scientific progress, was clearly enunciated.
According to a tradition which early Muslim scholars loved to quote, L Aristotle had inscribed above the door of his house: "Let no one enter who does not have a knowledge of mathematics." This science, together with logic, its handmaiden, was seen as the basis of all others and al-Farabi, the great Arab philosopher, who died in 950, placed logic and mathematics near the head of his Catalog of Sciences - a book which in its Latin translation had a considerable influence upon the curricula of medieval European universities. He arranged the sciences as follows : (1) the linguistic sciences (2) logic (3) mathematics (4) physics (5) metaphysics (6) politics (7) jurisprudence and (8) theology.
Accordingly, al-Hajjaj ibn Yusuf ibn Matar, who accompanied the first embassy to the Byzantine court, brought back a copy of Euclid's Elements and made two translations, one for the Caliph Harun al-Rashid and the other for al-Ma'mun. These translations served as the basis for a critical edition prepared by two of the most famous translators associated with the House of Wisdom, Ishaq ibn Hunain and Thabit ibn Qurra. Muslim scholars also translated a commentary to Euclid by Hero of Alexandria, the third century B.C. inventor and mathematician, who developed a prototype of the steam engine.
This was not the only work on Euclid to find its way into Arabic. Translations were also prepared of works either by Euclid or attributed to him on the subjects of optics, music, ethics, logic and weights and measures. The foundations laid by these translations of Euclid were buttressed not only by the translation of Hero's commentaries, but by at least 11 major works by Archimedes, including a treatise on the construction of a water-clock. Nichomachus of Gerasa (Jerash) had written a book on number theory in the second century, heavily influenced by Pythagorean theories, and this provided the basis for some of the more arcane Islamic speculations in this field. Other late classical mathematicians, men like Theodosius of Tripoli, Apollonius of Perga, Theon and Menelaus were also translated into Syriac and/or Arabic by the staff of the House of Wisdom.
Armed with these translations, as well as certain Indian works, the great age of Islamic mathematical speculation began. Its early development was intimately linked with two other disciplines based upon it: astronomy and music, or the science of harmonics.
The first great advance on the inherited mathematical tradition was the introduction of Arabic numerals. Scholars working at the House of Wisdom first became aware of them in translations of Indian astronomical works, and hence called them "Indian." These numerals embodied the place-value theory which allowed numbers to be expressed by nine figures plus zero (Arabic sifr, "cipher") and not only simplified calculation of all sorts but made possible the development of algebra.
Muhammad ibn Musa al-Khwarizmi, born in the town now called Khiva, seems to have been the first to systematically explore their use in his book, Addition and Subtraction in Indian Arithmetic, later translated at Toledo into Latin under the title Algorismi de numero indorum and introduced as "Arabic numerals" into the West. Al-Khwarizmi used both Greek and Indian sources and their cross fertilization led to his famous Kitab al-Jabr wa al-Muqabala, the first book on algebra; the word "algebra" is derived from the second word in his title and originally meant "bone-setting." Al-Khwarizmi used it as a graphic description of one of the two operations he uses to solve quadratic equations.
Interest in geometry began with the translation, as we have seen, of Euclid's Elements. The Islamic world responded to geometry even more whole-heartedly than it had to algebra, as the beautifully drawn geometric proofs which adorn the pages of Arabic manuscripts on the subject attest. Its study influenced both architecture and the decorative arts and Ibn Khaldun recommended the study of geometry as good training in logical thought:
Geometry is useful because it enlightens the intelligence of the man who cultivates it and gives him the habit of thinking exactly. Indeed, all the geometrical proofs are characterized by the clarity of their arrangement and by the evidence of their systematic order. That order and that arrangement make it impossible for any error to creep into the argument. Therefore the minds of people who are engaged in these studies are not in danger of being deceived and their intelligence is sharpened.
The great North African historian, in striking and homely image, goes on to say that the study of mathematics in general "is like soap for the clothes, which washes away the dirt and cleans the spots and stains."
The men most responsible for encouraging the study of geometry were the sons of Musa ibn Shakir, al-Ma'mun's court astronomer. These three men - Muhammad, Ahmad and al-Hasan - devoted their lives and fortunes to the quest for knowledge. Their devotion to the cause of science is all the more remarkable by virtue of the fact that they were private citizens; their interest in these matters shows how widely the scientific renaissance of the ninth century reached. Ibn al-Nadim says of them:
These men were some of those who took extreme pains to study the ancient sciences, for the sake of which they gave generously what was required, taxing themselves with fatigue. They dispatched to the Byzantine country men who sent scientific manuscripts back to them.
They hired translators from various districts and kept them in attendance for many years, so that they brought to light wonders of learning. The sciences in which they were most interested were geometry, mechanics, dynamics, music and astronomy.
The "Banu Musa", or "Sons of Musa," as they were called, not only spon-. sored translations of Greek works, but wrote a series of important original studies of their own: The impressive title of one of their works by Muhammad Ibn Musa reads: The Measurement of the Sphere, Trisection of the Angle, and Determination of Two Mean Proportionals to Form a Single Division Between Two Given Quantities. His interests were not limited to geometry, however; he also wrote works on celestial mechanics, the atom, the origin of the earth, and an essay on the Ptolemaic universe. His brother Ahmad wrote a fundamental work on mechanics, while al-Hasan wrote a study of the geometrical properties of the elipse. Al-Hasan was perhaps the most gifted geometrician of his time. He translated the first six books of Euclid's Elements and is said not to have finished it because he was by then able to work out the remaining propositions himself.
In terms of influence on mathematics in the West, the most important work of the Banu Musa was On the Measurement of Plane and Spherical Figures, which was translated in the 12th century by Gerard of Cremona under the title Verba filiorum Moysi filii Sekir, id est Maumeti, Hameti, Hasan.
The Banu Musa served a number of caliphs and occasionally were even involved in practical projects such as the construction of a canal. They were also famous for discovering perhaps the greatest of the scholars of the ninth century, Thabit ibn Qurra. While returning from a trip to Byzantium in search of manuscripts, Muhammad ibn Musa stopped in the town of Harran, where he met Thabit ibn Qurra, working as a money changer. Muhammad was so struck by Thabifs mastery of Syriac, Greek and Arabic that he persuaded Thabit to go to Baghdad - where such talents would find a suitable reward. There, Muhammad personally presented his protege to the Caliph al-Mu'tadid, who was so struck in his turn by Thabifs learning and intelligence that he appointed him court astrologer.
To the small coterie of scholars at the House of Wisdom, Thabit was invaluable, if only because his knowledge of Greek and Syriac was unrivaled. This latter language was important, for in many cases the writings of the Greek scientists were either preserved in Syriac versions, made by Nes-torian scholars of Iraq and Persia, or more frequently translated first into Syriac and then into Arabic. The reason for this was that the Christian communities, whose language was Syriac, tended to know Greek but not Arabic, while Muslim scholars found it easier to acquire a knowledge of Syriac, which is closely related to Arabic, than they did to learn Greek.
Most early translations prepared under the auspices of the House of Wisdom were done in this way, through the cooperation of teams of scholars of different religious and linguistic backgrounds. Thabits success was due as much to his linguistic abilities in the three major languages as to his very great natural gifts.
Thabit immediately set about correcting some of the earlier translations of important works, such as Ishaq ibn Hunain's editions of Ptolemy's Almagest and Euclid's Elements. His translations of key works by Archimedes, such as the famous Measurement of the Circle, were done into Latin in the 12th century by the indefatigable Gerard of Cremona, a worthy successor to Thabit.
Thabit also wrote more than 70 original works in the fields of mathematics, astronomy, astrology, ethics, mechanics, music, medicine, physics, philosophy and the construction of scientific instruments. He wrote valuable commentaries on Aristotle, Ptolemy and Euclid, as well as a series of introductions to other Greek thinkers.
Finally, his sons formed a dynasty of scholars that lasted to the end of the 10th century. His son Sinan was the most famous physician in Baghdad, director of several hospitals and court physician to three successive caliphs. He was an author as well, and wrote books on history, mathematics, astronomy and politics.
His son, Ibrahim ibn Sinan ibn Thabit, was also a prominent scientist, perhaps better known as, an instrument maker. When Ibrahim was 17 years old, he first became interested in various ways of reckoning time by the sun, and wrote a systematic treatise on the construction of sundials which remained standard for many years. One of Ibrahim's brothers, Thabit, named after his grandfather, was director of various hospitals, as well as the author of a work on history.
Thabit ibn Qurra and his descendants, together with the Banu Musa, led lives of extraordinary dedication to science and were enormously productive. The effect of both their translations and their original works - on their own and succeeding generations of scholars-was pervasive.
The Banu Musa and Thabit ibn Qurra and his sons did not work in isolation. The works that issued from the House of Wisdom were the product of many different men - linguists, editors, researchers, scribes and technical advisors. We unfortunately know little about how the House of Wisdom was organized. But we do know that these scholars developed certain academic techniques, such as collating as many different manuscripts of a given work as possible in order to establish a critical text, glossaries, annotations written in the margin of the page and the compilation of dictionaries of technical terms. These techniques are still basic to all academic research. Ibn al-Nadim lists 57 translators who were associated with the House of Wisdom and says that the running costs of the organization, including maintenance, came to 500 gold dinars a month.
Two other men also played critical roles in the transmission of Greek learning to the Muslim world: Hunain ibn Ishaq - known to the Latin west as Joanitius - and Qusta ibnLuqa, from Baalbek.
Born near al-Hira, the old capital of the Lakhmid dynasty in Iraq, Hunain ibn Ishaq was the son of an apothecary, who, recognizing his son's bent for medical studies, sent him to Baghdad. In the capital, Hunain found powerful supporters in the Banu Musa and set about learning Greek, and was soon translating the entire canon of Greek medical works into Arabic - including Galen, Hippocrates and the famous Hippocratic oath, obligatory then for Muslim physicians as it is everywhere today. Hunain was in many ways the most gifted of the translators associated with the House of Wisdom. His scholarly methods were impeccable, and he tended to translate more freely than many of the others, whose translations tended to err on the side of literalism - sometimes to the point of virtual incomprehensibility by those who did not know the original text.
Hunain also wrote at least 29 original treatises on medical topics. The most significant of these was a collection of 10 essays on ophthalmology. This work covers, in systematic fashion, the anatomy and physiology of the eye and the treatment of various diseases which afflict the vision. It is the first Arabic medical work to include anatomical drawings, and those that illustrate surviving manuscripts are very accurately drawn. This book was translated into Latin and for centuries remained the authoritative treatment of the subject in both Western and Eastern universities.
Hunain lived a life of exemplary piety and by his example did much to lend dignity to the medical profession. The Cdliph al-Mutawakkil, seeking to test Hunain's integrity, ordered him to prepare a poison; "I have learned only the actions of beneficial drugs, confident that this is all that the Commander of the Faithful would want of me," replied Hunain, and was rewarded by being made the director of the House of Wisdom.
Qusta ibn Luqa was also an accomplished translator and scholar. Ibn al-Nadim, in fact, considered him an even better translator than Hunain, and says: "He was never subject to criticism, being a master of literary style in the Greek tongue and excelling also in Arabic diction." Qusta wrote some 40 original works on an intriguing variety of subjects: politics, medicine, "burning mirrors," insomnia, paralysis, diseases which affect the hair, fans, the cause of wind, an introduction to logic, a book of anecdotes about the Greek philosophers, dyes, nutrition, an introduction to geometry, astronomy and "The Bath," to mention only a few.
Yuhanna ibn Masawaih was one of the early directors of the House of Wisdom. He served under four caliphs - al-Ma'mun, al-Mu'tasim, al-Wathiq and al-Mutawakkil. He wrote almost exclusively about medical problems, in particular gynecol-ogy. Ibn al-Nadim related the following anecdote, which shows that the scholarly milieu of ninth century Baghdad was not unrelievedly serious:
Ibn al-Hamdun, the court companion, made fun of Ibn Masawaih in the presence of al-Mutawakkil, where upon Ibn Masawaih said to him, 'If in the place of your ignorance there were intelligence, it could be divided among a hundred black beetles so that each one of them would be more intelligent than Aristotle.
Perhaps the greatest of the ninth century physicians was Abu Bakr Muhammad ibn Zakariya al-Razi, from the important Iranian town of Rayy. Al-Razi, known to the West as Rhazes, wrote, according to a bibliography of his writings compiled by al-Biruni in the 11th century, 184 works. Fifty-six of these dealt with medical topics. Al-Razi was»deeply versed in the classical medical tradition, as it had been made accessible in the translations that poured forth from the House of Wisdom, but his originality lay in his open advocacy of experiment and observation.
The authority of the Greek philosophers and scientists was so great that lesser men were content to accept their views without question. Not al-Razi, who questioned everything, and relied more on his own observations than on received attitudes. His gigantic compendium called al-Hawi, "The All-Encompassing," contains al-Razi's daily observations and diagnoses. He wrote a very important work on smallpox and measles, in which he correctly differentiates their symptoms for the first time.
A friend of Ibn al-Nadim gave the following lively account of al-Razi L at the height of his powers:
When I questioned a man, one of the people of Rayy, of great age, about al-Razi, he said: 'He was an old man with a large sack-shaped head, who used to sit in his clinic with students around him...a patient would enter and describe his symptoms to the first persons who met him. If they had knowledge of what was wrong, good; but if they did not, he would pass from them to others. Then, if they hit upon the diagnosis, good; but if not, al-Razi himself would discuss the case. He was generous, distinguished and upright with the people. He was so kindly, compassionate with the poor and the sick that he used to bring them substantial rations and provide nursing for them...He was never found when not taking notes or transcribing them, whether to make a rough draft or a revised copy'.
It is impossible to give an adequate idea of the range of al-Razi's thinking, even in the field of medicine (he was a philosopher and mathematician as well as a physician) but two titles give us a sense of the man's wit and common sense: The Reason Why Some Persons and the Common People Leave a Physician Even if He Is Clever and A Clever Physician Does Not Have the Power to Heal All Diseases, For That Is Not Within The Realm of Possibility.
Unlike their modern counterparts, these Muslim scholars did not specialize. They investigated any subject that interested them, for they regarded all fields of knowledge as essentially one. Perhaps the best illustration of this is al-Kindi, "The Philosopher of the Arabs," of whom Ibn al-Nadim says: "He was the most distinguished man of his time and unrivaled during his period for his knowledge of the ancient sciences as a whole."
Al Kindi was the first Muslim philosopher to show that there was . no essential conflict between Greek rationalism and Revelation. He was profoundly religious, and sought to use Aristotelian logic to support essential Islamic dogmas. But what is astonishing about al-Kindi is the range and depth of his speculations. He wrote about logic, philosophy, geometry, calculation, arithmetic, music, astronomy and a great many other things. He wrote an introduction to arithmetic as well as an almost endless list of important works: The Use of Indian Arithmetic; That the Sphere Is the Largest of Bodily Forms and That the Circle is the Greatest of All Plane Shapes; That the Surface of the Sea is Spherical; Calculating the Azimuth on a Sphere; An Introduction to the Art of Music; Projection of Rays; An Explanation of the Cause of the Retrogression of the Stars; The Reason Why Rain Rarely Falls in Certain Places; Areas of Vaulted Chambers; How to Form a Circle Equal to the Surface of A Designated Cylinder; Determination of the Hours on a Hemisphere by Means of Geometry; The Cause of Vertigo; The Reason Why the Highest Part of the Sky is Cold, While the Part Near the Earth is Warm; The Reasons for Cloud Formations; Calculation and Making an Instrument to Determine the Distances of Heavenly Bodies, Crossbreeding the Dove, Species of the Bee - and more.
Al-Kindi, and to a certain extent, al-Farabi, his successor, demonstrate the liveliness of Muslim thought as the 10th century drew to a close. Al-Farabi wrestled with many of the same philosophical problems as al-Kindi and wrote a book entitled The Perfect City, which expresses the degree to which Islam had first assimilated Greek ideas and then impressed them with its own indelible stamp. The Perfect City is an essay on what might be called ethical urbanism - the ideal city should be founded on moral and religious principles, and from there would flow the physical infrastructure. Al-Farabi undoubtedly had the magnificent round city of Baghdad, The City of Peace, in mind, which was consciously constructed on the pattern of the ancient cosmological cities of the east, its round form representing the Cosmos and its four gates the cardinal points of the compass.
With the death of al-Farabi in A.D. 950, the first period of Islamic scientific thought drew to a close. It had begun in 763 with the foundation of Baghdad; it had seen first the translation of the intellectual patrimony of the ancient world into Arabic, and then the first attempts to enlarge the intellectual horizons of that inheritance. Practically, the same period witnessed the development of certain basic social institutions to a very high point — hospitals, universities, libraries, charitable institutions and public services, such as the post and water supply. During the next 300 years, although the political empire of the Abbasids would slowly fragment, the intellectual and scientific progress would continue, although now centered in provincial centers - particularly Khorasan and Spain.
A popular anecdote illustrates the intellectual background of the times. The inventor of the game of chess was granted a single request by the ruler to whom the game was first presented. The inventor's request was simple. He wanted as many grains of wheat as would result if one placed one grain on the first square of the board, two on the second, four on the third, eight on the fourth, and so on until the 64th square of the chess board. The ruler agreed to grant what seemed a modest request, but when he came to fulfill it, he discovered to his chagrin that the chess board would contain all the grain in the kingdom.
Al-Biruni, an 11th-century Persian scholar, wanted to know exactly how many grains of wheat were involved in this problem. He arrived at the figure 18,446,744,073,709,551,615, and anyone who thinks medieval computational methods primitive, should try to solve this problem without the use of a calculator.
Al-Biruni accompanied Mahmud of Ghazna's famous expedition against India in 1001. While there he learned Sanskrit and wrote a History of India based on native sources and his own observations. Al-Biruni's accuracy in determining the number of grains of wheat in the chessboard problem is reflected in his historical work. Like his predecessors in Baghdad, he reveals both wide-ranging interests and a concern with practical problems. For example, he is the first known writer to identify certain geologic facts, such as the formation of sedimentary rock. He was a great mathematical astronomer and was centuries ahead of his time in criticizing the Ptolemaic model of the universe.
Al-Biruni was also the author of a most detailed treatment of spherical trigonometry. Trigonometry is in fact an innovation of Muslim mathematicians, who were the first to clearly define the sine, cosine and cotangent functions. Other mathematicians, such as Nasir al-Din al-Tusi, also the author of an important work on ethics, greatly advanced mathematical theory in allbranches, and'Umaral-Khayyam, better known in the West as a poet, wrote the clearest and most elegant textbook of algebra ever produced.
Many of these advances took place as a spin-off of the consuming . interest in astronomy so characteristic of Muslim lands at the time. Observatories were everywhere, and both physical and mathematical models of the universe were produced, and tables giving the distances of the fixed stars and the planets were continually refined. The size of the earth was measured to a degree of accuracy not attained again until the present century. The Muslim world, however, never abandoned the earth-centered theory of the universe which it had inherited from the Greeks.
In physics, al-Biruni and his compatriot, 'Umar al-Khayyam, both wrote on the subject of specific gravity and developed formulae for determining both the specific and the absolute weight of any object. The interest in mirrors and lenses which had engaged some scholars associated with the House of Wisdom led to sophisticated theories of optics. Ibn al-Haitham, who wrote in the 10th century, was perhaps the greatest Muslim scientist to devote himself to optics. He was the author of the most important book on the subject, The Book of Optics, in which he gives a detailed treatment of the anatomy of the eye. He rejected the classical notion that rays issue from the eye, and correctly stated that instead the eye receives light from the object perceived.
The inventiveness of later Muslim thinkers was turned to practical fields such as agriculture and irrigation. Ibn al-Haitham had proposed a plan to dam the Nile as early as the 10th century, and although this project had to wait until the 20th century to be realized, other, less ambitious projects were common. Dams, reservoirs and acqueducts were constructed throughout the Islamic world and some of these systems survive to this day. Muslim engineers perfected the water wheel, and developed many different kinds, powered by man, animals, wind, river and tide.
Well-digging and the construc tion of the elaborate under ground water systems called qanat required a high degree of engineering skill. Some of these qanat are as much as 15.5 meters (50 feet) deep and they were built with a very slight inclination over a long distance in order to tap underground water. They were provided with manholes so that they could be cleaned and repaired. By being placed underground they reduced water-loss through evaporation to a minimum.
Agriculture was dependent in much of the Middle East on irrigation, and a series of important books were written on soil analysis, water and what kinds of crops were suited to what soils. The passion for new plants, both for nutritive and medicinal purposes, led to widespread plant introductions: cotton, rice, mulberry trees, citrus fruits, cherries, all of which were adapted to new soils and climates in their spread from the East to the West. The technique of grafting was carried to a high art, particularly in North Africa and Spain.
Zoology and Botany were both actively cultivated sciences, and works like al-Damiri's Lives of the Animals contain much interesting material. In the field of botany, Abu Hanifa al-Dinawari, a 10th century scholar, made notable contributions.
Throughout the classical period of Islam, intellectual activity in every field was vigorous, first in Baghdad, later in Cairo and the regional capitals of Anatolia, Iran and, still later, in India. The Arabs accepted the classical heritage, fertilized it with the thought of India and the East and elaborated, criticized and corrected it; they then passed it on to the West where it formed the basis for the great technological achievements that have since transformed the world.