To Explain the World: The Discovery of Modern Science (17 page)

The divorce of medicine from philosophy did not last. In the following century the physician Ibn Bajjah (Avempace) was born in Saragossa, and worked there and in Fez, Seville, and Granada. He was an Aristotelian who criticized Ptolemy and rejected Ptolemaic astronomy, but he took exception to Aristotle’s theory of motion.

Ibn Bajjah was succeeded by his student Ibn Tufayl (Abubacer), also born in Muslim Spain. He practiced medicine in Granada, Ceuta, and Tangier, and he became vizier and physician to the sultan of the Almohad dynasty. He argued that there is no contradiction between Aristotle and Islam, and like his teacher rejected the epicycles and eccentrics of Ptolemaic astronomy.

In turn, Ibn Tufayl had a distinguished student, al-Bitruji. He was an astronomer but inherited his teacher’s commitment to Aristotle and rejection of Ptolemy. Al-Bitruji unsuccessfully attempted to reinterpret the motion of planets on epicycles in terms of homocentric spheres.

One physician of Muslim Spain became more famous as a philosopher. Ibn Rushd (Averroes) was born in 1126 at Córdoba, the grandson of the city’s imam. He became
cadi
(judge) of Seville in 1169 and of Córdoba in 1171, and then on the recommendation of Ibn Tufayl became court physician in 1182. As a medical scientist, Ibn Rushd is best known for recognizing the function of the retina of the eye, but his fame rests chiefly on his work as a commentator on Aristotle. His praise of Aristotle is almost embarrassing to read:

[Aristotle] founded and completed logic, physics, and metaphysics. I say that he founded them because the works written before him on these sciences are not worth talking about and are quite eclipsed by his own writings. And I say that he completed them because no one who has come after him up to our own time, that is, for nearly fifteen hundred years, has been able to add anything to his writings or to find any error of any importance in them.
⁵

The father of the modern author Salman Rushdie chose the surname Rushdie to honor the secular rationalism of Ibn Rushd.

Naturally Ibn Rushd rejected Ptolemaic astronomy, as contrary to physics, meaning Aristotle’s physics. He was aware that Aristotle’s homocentric spheres did not “save the appearances,” and he tried to reconcile Aristotle with observation but concluded that this was a task for the future:

In my youth I hoped it would be possible for me to bring this research [in astronomy] to a successful conclusion. Now, in my old age, I have lost hope, for several obstacles have stood in my way. But what I say about it will perhaps attract the attention of future researchers. The astronomical science of our days surely offers nothing from which one can derive an existing reality. The model that has been developed in the times in which we live accords with the computations, not with existence.
⁶

Of course, Ibn Rushd’s hopes for future researchers were unfulfilled; no one ever was able to make the Aristotelian theory of the planets work.

There was also serious astronomy done in Muslim Spain. In Toledo al-Zarqali (Arzachel) in the eleventh century was the first to measure the precession of the apparent orbit of the Sun around the Earth (actually of course the precession of the orbit of the Earth around the Sun), which is now known to be mostly due to the gravitational attraction between the Earth and other
planets. He gave a value for this precession of 12.9" (seconds of arc) per year, in fair agreement with the modern value, 11.6" per year.
⁷
A group of astronomers including al-Zarqali used the earlier work of al-Khwarizmi and al-Battani to construct the
Tables of Toledo
, a successor to the
Handy Tables
of Ptolemy. These astronomical tables and their successors described in detail the apparent motions of the Sun, Moon, and planets through the zodiac and were landmarks in the history of astronomy.

Under the Ummayad caliphate and its successor, the Berber Almoravid dynasty, Spain was a cosmopolitan center of learning, hospitable to Jews as well as Muslims. Moses ben Maimon (Maimonides), a Jew, was born in 1135 at Córdoba during this happy time. Jews and Christians were never more than second-class citizens under Islam, but during the Middle Ages the condition of Jews was generally far better under the Arabs than in Christian Europe. Unfortunately for ben Maimon, during his youth Spain came under the rule of the fanatical Islamist Almohad caliphate, and he had to flee, trying to find refuge in Almeira, Marrakesh, Caesarea, and Cairo, finally coming to rest in Fustat, a suburb of Cairo. There until his death in 1204 he worked both as a rabbi, with influence throughout the world of medieval Jewry, and as a highly prized physician to both Arabs and Jews. His best-known work is the
Guide to the Perplexed
, which takes the form of letters to a perplexed young man. In it he expressed his rejection of Ptolemaic astronomy as contrary to Aristotle:
⁸

You know of Astronomy as much as you have studied with me, and learned from the book
Almagest
; we had not sufficient time to go beyond this. The theory that the spheres move regularly, and that the assumed courses of the stars are in harmony with observation, depends, as you are aware, on two hypotheses: we must assume either epicycles, or eccentric spheres, or a combination of both. Now I will show that each of these two hypotheses is irregular, and totally contrary to the results of Natural Science.

He then went on to acknowledge that Ptolemy’s scheme agrees with observation, while Aristotle’s does not, and as Proclus did before him, ben Maimon despaired at the difficulty of understanding the heavens:

But of the things in the heavens man knows nothing except a few mathematical calculations, and you see how far these go. I say in the words of the poet
⁹
“The heavens are the Lord’s, but the Earth he has given to the sons of man”; that is to say, God alone has a perfect and true knowledge of the heavens, their nature, their essence, their form, their motion, and their causes; but He gave man power to know the things which are under the heavens.

Just the opposite turned out to be true; it was the motion of heavenly bodies that was first understood in the early days of modern science.

There is testimony to the influence of Arab science on Europe in a long list of words derived from Arabic originals: not only algebra and algorithm, but also names of stars like Aldebaran, Algol, Alphecca, Altair, Betelgeuse, Mizar, Rigel, Vega, and so on, and chemical terms like alkali, alembic, alcohol, alizarin, and of course alchemy.

This brief survey leaves us with a question: why was it specifically those who practiced medicine, such as Ibn Bajjah, Ibn Tufayl, Ibn Rushd, and ben Maimon, who held on so firmly to the teachings of Aristotle? I can think of three possible reasons. First, physicians would naturally be most interested in Aristotle’s writings on biology, and in these Aristotle was at his best. Also, Arab physicians were powerfully influenced by the writings of Galen, who greatly admired Aristotle. Finally, medicine is a field in which the precise confrontation of theory and observation was very difficult (and still is), so that the failings of Aristotelian physics and astronomy to agree in detail with observation may not have seemed so important to physicians. In contrast, the
work of astronomers was used for purposes where correct precise results are essential, such as constructing calendars; measuring distances on Earth; telling the correct times for daily prayers; and determining the
qibla
, the direction to Mecca, which should be faced during prayer. Even astronomers who applied their science to astrology had to be able to tell precisely in what sign of the zodiac the Sun and planets were on any given date; and they were not likely to tolerate a theory like Aristotle’s that gave the wrong answers.

The Abbasid caliphate came to an end in 1258, when the Mongols under Hulegu Khan sacked Baghdad and killed the caliph. Abbasid rule had disintegrated well before that. Political and military power had passed from the caliphs to Turkish sultans, and even the caliph’s religious authority was weakened by the founding of independent Islamic governments: a translated Ummayad caliphate in Spain, the Fatimid caliphate in Egypt, the Almoravid dynasty in Morocco and Spain, succeeded by the Almohad caliphate in North Africa and Spain. Parts of Syria and Palestine were temporarily reconquered by Christians, first by Byzantines and then by Frankish crusaders.

Arab science had already begun to decline before the end of the Abbasid caliphate, perhaps beginning about AD 1100. After that, there were no more scientists with the stature of al-Battani, al-Biruni, Ibn Sina, and al-Haitam. This is a controversial point, and the bitterness of the controversy is heightened by today’s politics. Some scholars deny that there was any decline.
¹⁰

It is certainly true that some science continued even after the end of the Abbasid era, under the Mongols in Persia and then in India, and later under the Ottoman Turks. For instance, the building of the Maragha observatory in Persia was ordered by Hulegu in 1259, just a year after his sacking of Baghdad, in gratitude for the help that he thought that astrologers had given him in his conquests. Its founding director, the astronomer al-Tusi, wrote about spherical geometry (the geometry obeyed by great circles on a spherical surface, like the notional sphere of the fixed stars), compiled astronomical tables, and suggested
modifications to Ptolemy’s epicycles. Al-Tusi founded a scientific dynasty: his student al-Shirazi was an astronomer and mathematician, and al-Shirazi’s student al-Farisi did groundbreaking work on optics, explaining the rainbow and its colors as the result of the refraction of sunlight in raindrops.

More impressive, it seems to me, is Ibn al-Shatir, a fourteenth-century astronomer of Damascus. Following earlier work of the Maragha astronomers, he developed a theory of planetary motions in which Ptolemy’s equant was replaced with a pair of epicycles, thus satisfying Plato’s demand that the motion of planets must be compounded of motions at constant speed around circles. Ibn al-Shatir also gave a theory of the Moon’s motion based on epicycles: it avoided the excessive variation in the distance of the Moon from the Earth that had afflicted Ptolemy’s lunar theory. The early work of Copernicus reported in his
Commentariolus
presents a lunar theory that is identical to Ibn al-Shatir’s, and a planetary theory that gives the same apparent motions as the theory of al-Shatir.
¹¹
It is now thought that Copernicus learned of these results (if not of their source) as a young student in Italy.

Some authors have made much of the fact that a geometric construction, the “Tusi couple,” which had been invented by al-Tusi in his work on planetary motion, was later used by Copernicus. (This was a way of mathematically converting rotary motion of two touching spheres into oscillation in a straight line.) It is a matter of some controversy whether Copernicus learned of the Tusi couple from Arab sources, or invented it himself.
¹²
He was not unwilling to give credit to Arabs, and quoted five of them, including al-Battani, al-Bitruji, and Ibn Rushd, but made no mention of al-Tusi.

It is revealing that, whatever the influence of al-Tusi and Ibn al-Shatir on Copernicus, their work was not followed up among Islamic astronomers. In any case, the Tusi couple and the planetary epicycles of Ibn al-Shatir were means of dealing with the complications that (though neither al-Tusi nor al-Shatir nor Copernicus knew it) are actually due to the elliptical orbits of planets and the off-center location of the Sun. These are
complications that (as discussed in
Chapters 8
and
11
) equally affected Ptolemaic and Copernican theories, and had nothing to do with whether the Sun goes around the Earth or the Earth around the Sun. No Arab astronomer before modern times seriously proposed a heliocentric theory.

Observatories continued to be built in Islamic countries. The greatest may have been an observatory in Samarkand, built in the 1420s by the ruler Ulugh Beg of the Timurid dynasty founded by Timur Lenk (Tamburlaine). There more accurate values were calculated for the sidereal year (365 days, 5 hours, 49 minutes, and 15 seconds) and the precession of the equinoxes (70 rather than 75 years per degree of precession, as compared with the modern value of 71.46 years per degree).

An important advance in medicine was made just after the end of the Abbasid period. This was the discovery by the Arab physician Ibn al-Nafis of the pulmonary circulation, the circulation of blood from the right side of the heart through the lungs, where it mixes with air, and then flows back to the heart’s left side. Ibn al-Nafis worked at hospitals in Damascus and Cairo, and also wrote on ophthalmology.

These examples notwithstanding, it is hard to avoid the impression that science in the Islamic world began to lose momentum toward the end of the Abbasid era, and then continued to decline. When the scientific revolution came, it took place only in Europe, not in the lands of Islam, and it was not joined by Islamic scientists. Even after telescopes became available in the seventeenth century, astronomical observatories in Islamic countries continued to be limited to naked-eye astronomy
¹³
(though aided by elaborate instruments), undertaken largely for calendrical and religious rather than scientific purposes.

This picture of decline inevitably raises the same question that was raised by the decline of science toward the end of the Roman Empire—do these declines have anything to do with the advance of religion? For Islam, as for Christianity, the case for a conflict between science and religion is complicated, and I won’t attempt a definite answer. There are at least two questions here.
First, what was the general attitude of Islamic scientists toward religion? That is, was it only those who set aside the influence of their religion who were creative scientists? And second, what was the attitude toward science of Muslim society?