(This five-part series on early Islamic history begins with the rise of Islam, shifts to its golden age, examines two major currents of early Islamic thought—rationalism and Sufi mysticism—and concludes with an epilogue. It builds on precursor essays I wrote at Stanford’s Green Library during a summer sabbatical years ago, and on subsequent travels in Islamic lands of the Middle East and beyond.)
Muslims discovered Greek thought hundreds of years before the Western Christians, yet it was the latter who eventually domesticated it. Why did the reverse not happen? Why did the golden age of Islam (approx. 9th-12th centuries)—led by luminaries such as al-Kindi, al-Farabi, Alhazen, al-Beruni, Omar Khayyam, Avicenna, and Averroës—wither away? Despite a terrific start, why did Greek rationalism fail to ignite more widely in Islam? In this epilogue, I’ll survey some answers that have been offered by historians and highlight one that I hold the most significant.
Earlier in this series, we saw how three contending currents of thought dominated the Islamic golden age—orthodoxy, rationalism, and mysticism—based on three different ways of looking at the world. Orthodoxy in Islam looked to the Qur’an to justify a whole way of life. A universal, durable code of behavior and personal conduct is an understandable human craving, and so much more comforting when God Himself shows up and lays it out in one’s own language! Orthodoxy is by no means limited to ‘revealed’ religions; it took root in Hinduism via its castes, priests, and rituals. Suffice it to say that humans have been drawn to narrow and exclusive systems of belief with a dismaying alacrity.  The orthodox, it’s worth pointing out, are not all that otherworldly. The mullahs, bishops, and pundits are rarely disengaged from their social milieu, as the mystics tend to be. The orthodox may covet the rewards of the other world but what happens in their own—as in what norms, practices, dogmas, and rituals are followed—is profoundly important to them. They care deeply about this world and, in their own way, struggle to improve it, sometimes even waging war over it.
The mystics are rather different. They don’t care much for holy books or religious clerics, and receive God as a subjective experience, beyond the bounds of dogma. An essential mystical experience lies in the believer’s sobering realization of the inadequacy of reason in knowing God and his design. Love and devotion—even rapturous ecstasy—help bridge the enormous gulf he sees between him and God. Happiness comes not from material pleasures but from surrendering to the benevolent divine. He deals with existential angst by suppressing his self and ego. Mystical teachers across cultures have appealed to a non-dualistic approach to nature, in which everything in existence is not only interwoven but is a manifestation of the divine. Clearly, a mystical worldview does not engender ideas like competition, personal ambition, or democracy, nor does it preoccupy itself with theories of justice or science or critical inquiry. Instead, it eschews religious orthodoxy and furthers a tolerant, pacifist, and private faith, often alongside a gentle, dreamy, fatalistic detachment from the world.  Such otherworldly mysticism flowered in Islam, Hinduism, Judaism, and Eastern Christianity, but barely so in Western Christianity.
The rational current of Islam, as also in Western Christianity, was sparked by a small group of philosophers, scientists, doctors, liberal theologians, explorers, poets, artists, and their royal patrons in a fortuitous window of peace, prosperity, and cosmopolitan culture. They drew inspiration from ancient Greek texts, adapting them to their own milieu. A great intellectual ferment led to notable advances in areas like navigation, astronomy, and mathematics, which became the bedrock for later European advances. The bold thinkers who led the Islamic golden age profoundly inspired later Christians, including Thomas Aquinas, Roger Bacon, John Duns Scotus, and William of Ockham. What then explains the fate of al-Farabi’s rationalist tradition? Before considering some answers, let’s briefly revisit the Greeks to remind ourselves of a few defining features of their society.
The three pillars of Greek identity were language, religion, and culture. Greek religion was derived from the ‘age of the heroes’ depicted in the Iliad and the Odyssey. Humans had descended directly from the gods, who numbered in the hundreds, inhabited the same natural world, and were variously patronized by human communities. A striking feature of the Greek gods is their remarkable humanization. Apart from their immortality and superior power, they were mirror images of people. At times, they too lusted, hated, cheated, seduced, schemed, turned jealous, and acted without scruples. They just had to be on your side, and if not, you had to cajole and bribe them with gifts and sacrifices—the gods grew fond of and helped individuals, no matter what their conduct. Mostly harmless, they didn’t stand in judgment from high above, and didn’t give anyone a hard time unless one challenged their enormous egos—like the mortal Paris inviting proof by judging Hera and Athena less beautiful than Aphrodite; the Trojans paid for his folly with their blood!
Though the average Greek was pious and superstitious, and could be penalized for not honoring the gods of the city, Greek religion had no orthodox doctrine, no ‘mother church’, no concept of sin or evil, and therefore, no sacrilege, no guilt, no atonement. Religion provided little moral guidance for everyday living. They were no mystics either—their Homeric paganism had little, if any, otherworldly spirituality.
The Greeks were, above all, creatures of the polis; their values were built upon the public space; morality derived from the logic and culture of the demos. In 5th century BCE, Pericles declared, ‘unlike any other nation, we regard him who takes no part in public duties not as unambitious but as useless.’  Perhaps for the first time in any culture, man became the measure of all things. The Greek elevation of the individual and their respect for public success and achievement helped encourage self-reliance, curiosity, and ambition. Reward was personal and immediate and came in the form of fame and glory. Competitive self-interest became the norm as it didn’t in any other society until two millenniums later. This is a worldview antithetical not only to the mystical one but also to the orthodox one. Indeed, any embrace of the Classical Greek worldview is at the expense of the other two almost by definition. 
Scholars have proposed many causes for the fizzling out of the Islamic golden age: (a) the ‘closing of the gates’ of ijtihad in Sunni Islam by the 12th century; ijtihad is independent reasoning or interpretation of scripture; (b) the intransigence of the orthodox; (c) the absence of a ‘mother church’ as a focal point for later reform; (d) insufficient separation between ‘church’ and state; (e) curricular inadequacies and the meager autonomy of medieval institutions of learning; (f) the absence of certain legal institutions, such as the early corporation of Europe, which had a big impact on collective, accretive pursuits, such as science; (g) new economic motives that funded scientific inquiry in the West (particularly the pursuit of resources and wealth from overseas colonies). All these and other historical contingencies surely played a part but some of these also strike me as the outcomes of other, more fundamental causes. 
I think the single biggest cause, perhaps even the decisive cause, which incidentally hasn’t received enough attention from historians, is that outside elite circles, conditions for the rise of individualism — the kind that defines the Greeks and which may well be necessary for rationalist inquiry and scientific innovation — didn’t quite exist in the lands conquered by the Arabs. For the long tradition of ascetic and spiritual life in the Abbasid heartland (also in Eastern Christianity and in Hinduism), the more natural development was Sufi mysticism, not the assertive self-expression of the Greeks. Mysticism, not surprisingly, became the dominant face of Islam until the 19th century (when orthodoxy began rising, partly in reaction to colonialism and modernity).
Islam, in other words, only had a good fling with rationalism, to which a widespread mysticism denied critical mass, withheld the oxygen required to fuel its spread. It is one thing for enlightened patrons to encourage intellectual creativity, another for it to spread organically. Lack of interest from the masses was a major culprit, and not, as many tend to assume today, the hostility of the orthodox, whose intransigence then was no greater in Islam than in Western Christianity. The same phenomenon is plainly evident in medieval India too, where the masses much preferred Hindu mysticism (bhakti) to Mahayana rationalism, which led the royalty and other elites to eventually abandon patronage for the latter. The mystic tide that also swept over India a millennium ago was not hostile to science and rationalism, but simply indifferent to it. The masses just didn’t care for it (deeper explanations surely lie in ontology), failing to provide a large enough substrate from which new crops of rationalist thinkers could sustainably emerge. It remains an open question whether long-term patronage and top-down action can succeed in building such a substrate over time, but in medieval Islam it surely didn’t happen long enough.
This contrasts with Western Christianity, which (unlike Eastern Christianity) lacked a significant mystical dimension in the faith of its masses.  The rise of individualism in the West is often traced to the 14th century Christian protests against ecclesiastical abuse, when egregious disparities appeared between preaching and practice in the Roman Church. Theologians like Aquinas had already furnished allegorical interpretations of scripture, harmonizing it with Plato and Aristotle. The Protestant Reformation now began fostering greater self-reliance in matters of faith, raised the scrutiny of biblical texts, and marginalized the relevance of the clergy. The Renaissance arrived with bold and confident artists, authors, architects, philosophers, and scientists. In reaching this stage, the West was half-a-millennium behind Islam. But from here on, their trajectories diverged: the West took the ball, so to speak, and ran with it.
While the Renaissance began in Italy, its most robust expression took place in parts of northwestern Europe that had led the Protestant Reformation—regions that had even poorer records of mystical spirituality and far greater obsession with the ‘fundamentals’ of scripture. Both the orthodox and the rationalists care deeply about this world. They pay great attention to the principles by which they understand it and conduct their public lives.  The proximity this creates—of cognitive investment (usually dualistic), literacy, social class—perhaps helps explain why it has been easier for people in history to cross over to rationalism from orthodox frames than from mystical ones.  Wasn’t the Bengal Renaissance of the 19th century, for instance, led mostly by the ranks of orthodox Brahmins turned rationalists?
Notably, science did not jar as badly with the Qur’an as it did with the Bible. The latter was built upon an elaborate creation myth and the divinity of a man, both of which barely stood a chance. The Qur’an could better withstand science because it relied only on a universal God with a human messenger, not His incarnation/son; and while the Qur’an invokes the Biblical creation myth, it does so sparsely, peripherally, and ambiguously (for e.g., not six days but six ‘youms’, each could be millions of years; there is no virgin birth and no resurrection). Christian metaphysics was therefore much more problematic for science (with Buddhist metaphysics much less so than both).
Inevitably, the new rationalism butted heads with Christian orthodoxy and began eroding the hold of religious dogma on many more people. Without the tether and pull of mystical spirituality among the masses, this fueled popular attitudes and values similar to those of Classical Greeks—including the abstractions of the autonomous self and individualism—which, as with the Greeks, turned out to be enormously fruitful for intellectual inquiry, science, and political institutions built via negotiation.  The rest, as they say, is history.
1. Sociobiology and game theory only offer partial explanations at best. See H. Allen Orr, Can Science Explain Religion?, NY Review of Books, Volume 57, Number 1 · January 14, 2010.
2. Unlike the orthodox Hindus of the upper castes, medieval Hindu mystics didn’t care for holy books and their devotional folk religiosity accommodated women, lower castes, the unlettered, and even Muslims.
3. Thucydides, History of the Peloponnesian war, The Funeral Oration of Pericles.
4. Jacob Burckhardt (1818-97), The Greeks and Greek Civilization; Translated by Shiela Stern (1998). A celebrity historian (Swiss), regarded by many as the father of cultural history. Arguably the best modern study on the Greeks and I recommend it highly.
5. See this interesting exchange of views between George Saliba and Toby Huff from a few years ago.
6. Protestant thinkers like Karl Barth and Rudolf Bultmann have also denied mysticism an integral role in Catholic belief, claiming that ‘mystical union was [an Eastern Christian] import incompatible with saving faith in the Gospel word.’ Karen Armstrong, a scholar of religion, has also supported this view: ‘In the West, Christians were slower to develop a mystical tradition. They had fallen behind the monotheists in the Byzantine and Islamic empires and were perhaps not ready for this new development...’
7. And when the orthodox and the rationalists start insisting on narrow and exclusive interpretations of reality—whether derived from ‘revelation’ or ‘reason’—they begin to really resemble each other.
8. It’s not that anyone can be purely orthodox, or a pure rationalist, or a pure mystic. People and cultures usually exhibit an ever-changing mix of all three. Like all similar classifications, these categories are more useful for analysis in the aggregate. In India, for instance, the mystical frame is still very widespread, explaining why scientific rationalism has failed to penetrate to the extent it has, despite modern education and progressive socialist policies from the top down. There is no hostility to science, mostly indifference. Science is accepted more as a vehicle for economic ends, less for its wider epistemological implications.
9. In this series on Hobbes’s Leviathan, Mary Midgley charts the rise of individualism in the West and its enormous impact on politics and society, both good and bad.
1. A conventional portrait of philosopher Al-Kindi, 801-873 CE (source).
2. A sketch of a whirling dervish, copyright unknown.
3. Aphrodite, Cupid, and Pan; Archaeological museum, Athens.
4. Bust of Pericles. Marble, Roman copy of a Greek original, ca. 430 BC (source).
5. Sources and influences of Muslim philosophy (source).
6. The Vaishnava meets the Sufi as Akbar looks on (source).
7. The Vitrurian Man, Leonardo da Vinci, 1487 CE.
8. The Great Mosque of Samarra, Iraq, built in the 9th century CE.