The tragic and talented Siger—one of his students called him “the most distinguished teacher of philosophy”27—never returned to the lecture hall. In truth, his views had never strayed far from those of Thomas, whom he had clearly read and admired, but his unyielding insistence that the philosophers go where reason took them, an early defense of intellectual freedom, cost him his career and possibly his life. A chronicle from his native region of Brabant tells us that he died at the hands of a crazed cleric: “This Siger, a Brabantine by birth, as a consequence of holding certain opinions against the faith, was no longer able to remain in Paris, and went to the Roman court, where after a short while he died of stabbing by his half-mad secretary.” The date of his murder must have been sometime before November 1284, when a letter from the archbishop of Canterbury makes mention of his death.28
The condemnations of 1277 dampened enthusiasm in Paris for rationalist speculation and natural philosophy, but they failed to weed out the influence of Thomas, or that of Averroes and his pugnacious acolyte, Siger de Brabant. The locus of scientific and philosophical activity in many cases simply shifted elsewhere, and the influence of the Averroist tendency took root as far away as Poland and England. Theology historically enjoyed little prestige or influence at the Italian universities, such as Padua and Bologna, and Averroes’s teachings flourished there into the seventeenth century. Even at Paris, it was not so long before such matters were again openly read and debated. The men of science were clearly here to stay.
It is tempting to attribute their success to the raw power of natural philosophy and to the inability of the church to stamp out this competing “theory of everything” in the same way it had destroyed the Cathar heresy. Yet to do so is to overlook the crucial role of the Arabs as master architects—not simply as midwives—of the emerging Western worldview. This was no mere “recovery” of classical wisdom by the medieval Latins, with the Arabs cast in the role of benevolent guardians, as most Western histories of the period tell us. Rather, it represented the enormous transfer—some might even say cultural theft—of invaluable Arab knowledge and technology directly to the Christian West.
The case of Aristotle’s natural philosophy is but one prominent example of the Arab influence at work. The great philosopher never had much time for God, and certainly did not conceive of a deity like the one who governed the monotheistic faiths of Judaism, Christianity, and Islam. The same could not be said of the medieval Arab philosophers, from al-Kindi to Averroes, who painstakingly subjected Aristotle to the demands of their belief in the one true God. And it was this “Arab Aristotle,” not so much the pagan thinker of classical Greece, who ultimately triumphed in the West. Once in place, much of this Aristotelian worldview—particularly its rigid, even doctrinaire, conception of the cosmos—would face centuries of critical study by Christian scholars, a reevaluation that ultimately would lead to something resembling modern science.
As it happened, a similar process had long since been under way in the lands of Islam.
Just as Avicenna and Averroes “corrected” Aristotelian metaphysics to make room for God, so from the eleventh century onward the Arab scientists—the successors to Baghdad’s House of Wisdom—assembled their own critical response to Greek astronomy and cosmology. The result was a theoretical and practical assault on the accepted architecture of the universe, codified by Ptolemy in the second century A.D. Gradually, the way was paved for that system’s complete overthrow, proposed by the Polish astronomer Nicolaus Copernicus in the mid-sixteenth century and completed by Isaac Newton 150 years later. This celestial revolution put the sun, rather than the earth, at the center of the universe and affirmed the predominant position of science in Western society.29 The transformation of man’s place in the heavens—from the center of attention to just one among many—required not only a profound psychological shift but also some high-powered scientific innovation. Here, too, the West got some vital help from the Arabs.
Specifically, the only “original” theorems in Copernicus’s monumental De Revolutionibus, published in 1543 as the scientist and churchman lay on his deathbed, have been traced directly to the earlier work of highly sophisticated Arab scientists unhappy with the teachings of the Almagest, Ptolemy’s great astronomical textbook. In the early years of Arab science, Abbasid scholars gently edited and revised this classic work. Supported by al-Mamun and some of the other early caliphs, these astronomers corrected Ptolemy’s calculation of the length of the solar month and greatly improved his measurement of the angle of the sun’s course around the earth, known as the ecliptic. Such initial changes, important but not fundamental to the underlying theory of the original work, were generally incorporated into updated Arabic translations of the Greek text.30
Other improvements to the Almagest were more significant, such as the introduction of the Arab trigonometric functions to replace or supplement the more cumbersome chords used in the Greek tradition. “I say, since the method of the moderns, which uses the sines at this point instead of the chords, is easier to use, as I will explain below, I wish to refer to it as well,” writes the astronomer Nasir al-Din Tusi in his Redaction of the Almagest in 1241.31 Such was the importance of this process that the medieval Latin translators got better results working with Arabic editions of the Almagest, rather than starting all over again with the unedited Greek original.
This gradualist approach was joined by more ambitious efforts to evaluate Ptolemy’s model of the universe on theoretical grounds. Here, the main sticking point was the Almagest’s readiness to violate, when necessary, one of the cardinal rules of natural philosophy, as taught by Aristotle and accepted by Ptolemy and his successors, including the Arabs: that celestial objects all moved in uniform circular motions, with the earth at their center. Ptolemy had already tried to account for the irregular movement of the celestial bodies with his notorious equant point, but he then shifted this theoretical axis of rotation away from the center of the earth—and thus from the center of the universe—in order to reflect centuries of observational data on how the planets actually moved when seen from the earth. By suggesting that some of these orbs effectively rotated around an axis that did not pass through the center of the universe, the Almagest introduced planetary motion that was neither perfect nor uniform.
This, said the early Arab critics, meant that Ptolemy’s account of planetary motions was “false,” giving rise to the theoretical literature known as shukuk, or “objections.”32 The oldest detailed critique dates to the mid-eleventh century, completed one hundred years before Hermann of Carinthia and Robert of Ketton struggled mightily in Spain to even understand the science of the Almagest well enough to translate it into Latin.
It was not long before the critique of Greek astronomy spread from the realm of science to that of natural philosophy. Avicenna took note of Ptolemy’s theoretical shortcomings, as did Averroes and Maimonides. These latter philosophers, along with Averroes’s mentor Ibn Tufayl and others, were part of a sustained critical tradition centered in al-Andalus that sought to replace the model of the Almagest with a nest of hollow spheres all centered on the earth.33 The effort failed—although Avicenna hinted that he had found a separate way to save the model by eliminating the offending equant point, a claim that even his most loyal student dismissed34—but it nonetheless reveals the extent to which the Arabs were demanding that science not only account for observed phenomena but also accord with its own understanding of reality. In other words, science had to be both predictive and consistent, central tenets of the modern scientific method. “The science of astronomy of our time contains nothing existent, rather the astronomy of our time conforms only to computation and not to existence,” complains Averroes.35
Astronomers connected with an observatory at Maragha, in what is today northwest Iran, produced a number of significant breakthroughs to address the deficiencies found in classical astronomy. This research center was built in 1259 on the orders of Genghis Khan’s grandson, Hulegu, who one year
earlier had led the Mongol forces in the sack of Baghdad and the execution of the last of the Abbasid caliphs. Baghdad’s authority had long since been whittled down from its once-great expanse, and the caliphs, whose religious significance remained important, had been reduced to little more than political figureheads by the Mamluk warlords who had once served them. But the end of empire did not extinguish the scientific traditions once animated by the early Abbasids. As with the diffusion of scholarly learning into scattered Muslim courts after the collapse of central rule in al-Andalus, so, too, did other centers in the East exhibit remarkable intellectual activity after the loss of Baghdad. These included Diyarbakir, in southeast Turkey, as well as Isfahan, Damascus, and Cairo.36 Such was the case at Maragha, which brought together an extraordinary collection of astronomers, engineers, and other experts and included a state-of-the-art science library.
Nasir al-Din Tusi, now the observatory’s director and scientific adviser to Hulegu, had already devised an ingenious approach to the problem of the equant, one that generated linear motion from the uniform rotations, in opposite directions, of two spheres. Modern scholars have dubbed this the Tusi Couple. Not only did this address a major shortcoming within Ptolemaic astronomy, but it also helped later Arab scientists, as well as later Western ones, mount serious challenges to the authority of Aristotelian physics.37 Meanwhile, Tusi’s assistant and the designer of the observatory’s specialized instruments solved the same problem in a different manner. Over time, the theorems developed by Tusi and his colleague were introduced into a range of planetary models by the Arab astronomers, most elegantly by the official timekeeper at the Umayyad mosque in Damascus, Ibn al-Shatir, who used them to account for the movements of the moon, the so-called upper planets, and the lower planet, Mercury.
Ibn al-Shatir died in 1375, but 168 years later his use of the theorems of the Maragha astronomers turns up in the groundbreaking work of Copernicus, suggesting that the Polish astronomer must have been familiar with the work of his Arab predecessors.38 No means of direct transmission has yet been established, and there is no evidence that Copernicus knew Arabic or that these theorems were ever published in Latin. There are only hints: From 1496 to 1503, he studied in Italy, where Arab science and philosophy avoided the backlash experienced in Paris; there were in his day a number of Western Arabists capable of explaining such advanced works to Latin scientists; and Copernicus, who had studied Greek, may also have had access to late Byzantine borrowings from Arab astronomy. Adding to the mystery, Tusi’s proof of his couple, made around 1260, and the proof included by Copernicus in his De Revolutionibus three centuries later use identical designations for the same geometric points, an indication to modern scholars that Copernicus had firsthand access to Tusi’s work.39
Neither Ibn al-Shatir nor Tusi ever suggested anything as radical as transposing the Ptolemaic model to place its center at or near the sun, the defining feature of what became known as the Copernican revolution, although some Greek and Arab scholars had already pondered the idea. The enormous obstacles facing any theory of a sun-centered universe—established religious teaching and philosophical tradition, common sense and daily human experience, and the lack of a gravitational theory to make the whole thing work—testify to the genius of Copernicus’s insight and to the brilliance of the Western men of science who later perfected his work. Yet it is worth nothing that Ibn al-Shatir had already imposed uniform circular motion on Ptolemy in such a way that all planetary movements now revolved around a single point, the earth. This made Copernicus’s conceptual breakthrough that much easier by allowing him to shift that center toward the sun without having to reinvent an entire model of the heavens from scratch.40
The scientific, philosophical, and theological struggles over Copernicus’s proposal of a heliocentric universe, which was buried in his famously difficult treatise under a mountain of complex mathematics, continued for many years.41 The birth pangs of the new world of independent science included Galileo’s heresy conviction in 1633 for his support of Copernicus, the earlier burning at the stake of the freethinking philosopher Giordano Bruno, and the persecution of countless others by the Catholic Church, at disastrous and lasting cost to its reputation and authority.
Nonetheless, the fearsome inquisitors never managed to put the jinn of Arab science back in the bottle. The findings of Johannes Kepler on elliptical planetary orbits and Isaac Newton’s later theory of gravitation, published in 1687, effectively completed the work of Copernicus and helped guarantee the success of the scientific revolution. The church was forced to abide by the verdict of natural philosophy, its former handmaiden, and accept that the earth in fact revolved around the sun. Galileo was eventually rehabilitated, and in 1979 Pope John Paul II expressed regret for the mistreatment of the great Italian scientist and inventor at the hands of the church.
The verdict of history on this entire episode has been harsh, and rightly so. This is all the more the case because the church willfully ignored the prescriptions of its own St. Thomas—and through him those of Averroes—for the peaceful and productive coexistence of faith and reason. Under the direct influence of the Arab Aristotelians, Thomas had carved out a truce between traditional church teachings and the discoveries of the emerging generations of modern Western scientists. That compromise defines the rules of engagement to this day between the realms of faith and reason. And it stakes the Arabs’ claim as inventors of the West, a debt that Adelard of Bath identified many centuries ago on his return from Antioch: “Of course God rules the universe,” he assures his readers. “But we may and should enquire into the natural world. The Arabs teach us that.”42
NOTE TO READERS
DEFINITIONS OF TERMS and concepts are rarely associated with works for the general reader, no matter how serious or weighty the subject, and I have deliberately kept these to a minimum. Nonetheless, a few words are in order at the outset about my choice of “Arab science”—or words to that effect—to convey the complex cultural milieu of the medieval Islamic world, rather than “Islamic science.” As many readers will already be aware, much of the cultural flowering in this time and place was not exclusively the work of ethnic Arabs. Nor was it strictly the work of Muslims. Persians—including Zoroastrians and Christians—Jews, Greeks, Syriac Christians, Turks, Kurds, and others played crucial roles in all aspects of science, theology, and philosophy.
However, this work was almost always conducted in Arabic and frequently under the aegis of Arab rulers, most notably the Umayyad and Abbasid caliphs, first in Damascus and then in Baghdad. In one notable case, as we shall see, an ethnic Persian scholar produced a major work in his native language but then rewrote it in Arabic, which he found far more precise and more effective for his purposes. Throughout much of the period in question, Arabic served as the global language of scholarship, and learned men of all stripes could travel widely and hold serious and nuanced discussions in this lingua franca. Medieval Western scholars who wanted access to the latest findings also needed to master the Arabic tongue, or work from translations by those who had done so. It is also worth noting that such labels, today largely associated with nation-states and the demands for distinct cultural identity, were far more fluid in the era under discussion.
This is not to say that Islam and the unique culture of the Muslims are not important elements of our story. I refer to the great importance of Islam to the development of Arab science throughout the text and have devoted an entire chapter to this vital relationship between faith and reason. Yet much of the research during this period went well beyond specific questions relating to the Islamic faith and was not generally carried out with an eye to establishing theological or doctrinal truths. At the same time, it is worth avoiding any confusion with the established notion of the “Islamic sciences,” which generally refers to strictly religious disciplines: jurisprudence, Koranic exegesis, the study of the hadith, or the collected sayings of the Prophet Muhammad, and so on.
A few words on my use of names
and dates and my system of transliteration will also be useful. This work presents the enormous impact of Arab learning on the West—that is, on the lands of medieval Christendom and the states and societies they later produced. It seemed only sensible to use the Latinized forms instead of the Arabic names in the case of figures widely known to the Western world. Thus, I have used the Latin Averroes, not the Arabic Ibn Rushd, and Avicenna, not Ibn Sina. Less familiar figures have retained their Arabic names. For similar reasons, dates are presented in the traditional “Western” fashion—that is, anno Domini (A.D.) and before Christ (B.C.) In transliterating, I have chosen readability, familiarity, and convention over linguistic purity or consistency.
Finally, a reference to the structure of The House of Wisdom, which pays tribute to the success of Arab scholars in measuring out the ever-changing pattern of night and day that determines the times of the five daily Muslim prayers. The book begins at sunset (al~maghrib prayer), the traditional start of the day in the Middle East; then moves through the nightfall (al~isha) of the Christian Middle Ages; recounts the dawn (al~fajr) of the great age of Arab learning; soars toward the glory of midday (al~zuhr) with our central hero, Adelard of Bath, in the Near East; and concludes with the rich colors of afternoon (al~asr) that mark the end of the Age of Faith in the West and the seemingly unstoppable triumph of Reason.
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