One of the questions people have been asking lately is the significance of the dominance of Islamic civilization during the Middle Ages and its subsequent decline. Some people claim that they know the answer. For example, Reza Aslan claims that religion is not to be blamed for the decline because it functions as nothing more than a Rorschach test, so that whatever people claim that they find in their religion to influence their behavior is actually instead a reflection of them and not of their religion. Some people assert that it is an indisputable fact that any decline can be attributed entirely to rapacious non-Muslims invaders who militarily conquered and then destroyed Islamic civilization. This is a factual, and therefore potentially empirically resolvable, question. We should look to evidence to measure if there was a decline over time to answer this question. Then we can align evidence of decline with contemporaneous events to see if we can uncover the most likely culprits via consistent chronological correlation.
The question now becomes one of how to measure the rise and fall of Islamic civilization using available historical data. Eric Chaney, an Economist at Harvard University, figured that he may be able to measure the success of Islamic civilization by counting the number of scientific books published over time by authors with Muslim names. Harvard University is famous for the size of its library which has many books from past centuries.
He identified 21,275 books written by 3,784 authors with Muslim names who died between the years 250 and 1799. When there was no death date he substituted birth date plus 69 years or initial publication date of the author's final book if there was no birth or death date. He grouped the statistic into century intervals. During the two centuries from 900 to 1100 an average of 12% of the published books were on scientific subjects and 30% on religious subjects. This was followed by a decline in the percentage of books on scientific subjects. By 1300 7% of the books cover a science subject and by 1700 it is 2% while the percentage books on religious subjects rose to 40%.
To better pinpoint the correlation of this decline in scientific output with other historical events, Chaney divided the results geographically into three regions: East (Chaney refers to this region as "Iran"), Levant, and West. The East region experienced the Mongol invasions, the Levant experienced the Crusader invasions, the West experienced the invasions of European colonialists. He found that the decline in publication of science subject books began in the East and spread from there to the Levant and subsequently to the West. The scientific decline in the East began a century before the Mongol invasion. The subsequent decline in the Levant and West similarly began centuries before the Crusader and European colonist conquests. The Levant experienced an anomalous increase in publication of science books (as a percentage of overall book publications) during the Crusader invasions.
In his draft study titled Religion and the Rise and Fall of Islamic Science, Eric Chaney concludes that the role of Islamic civilization in world affairs correlates best with the relative dominance of the rationalist versus the traditionalist strains of Islam. The Mu`tazila movement was one example of rationalist Islam. They believed that good and evil were not determined by revealed scripture or interpretation of scripture, but are rational categories that could be established through unaided reason. These rationalist movements first emerged in the preceding Umayyad Era and reached their height in the Abbasid period. During Islam's "Golden Age" the rulers of the Abbasid Caliphate forceably favored a rationalist Islam over the competing traditionalist Islam, they tortured traditionalists. The Abbasid Caliphate systematically translated every available science text into Arabic (their motive may have been to equip theologians with knowledge to win debates for the purpose of converting people to Islam). After the 10th century the Islamic rationalist movement declined.
Why did the Islamic rationalist movement lose its influence? One possible reason is that they had angered people by being extreme in their intolerance of Muslims with opposing beliefs and by linking themselves to the brutality of the rulers. Another reason was a demographic change. During the Abbasid period the population of large parts of the Caliphate were non-Muslim. Muslims were initially a minority of citizens overall. This changed as a result of conversions to Islam with Muslims becoming the majority first in what is now Iran and Iraq, later in the Levant, and then in the West. By the start of the eleventh century the Abbasid Caliphate had switched sides, outlawing rationalist Islam as heresy and endorsing traditionalist Islam (by then the Caliphate was weak and had little temporal power). The number of madrasas, which are schools of Islam, increased. Traditionalist Islam, which emphasizes the importance of revelation and opposes rationalist Islam, became increasingly dominant. The number of authors employed by government declined and the number employed by madrasas increased. The relatively more secular state bureaucracy that pre-dated Islam was replaced by Islamic religious institutions.
Although a partial dose of rationalism was advantageous for converting non-Muslims to Islam, a full dose was corrosive towards religious faith, promoting skepticism, agnosticism, deism, and atheism. That is why it was abandoned. Today rationalist Islam is often deemed heretical by scholars in mainstream Islamic theology.
Islam itself contained the anti-rationalist seeds that contributed to the diminished role of Islam in world affairs. Contrary to what apologists for Islam say, there is good reason to think that the religion of Islam has been, and continues to be, its own worst enemy. No foreign interventions caused this weakness. The weakness resulted from rationalism being disfavored when the influence of Islam increased following a majority of citizens self-identifying as Muslim. A diminishment of the negative influence of traditionalist Islam's anti-rationalism is the way forward. That is also a good prescription for everyone, everywhere. A good place to start is a willingness to abandon religion altogether. We need fewer people who fear not being religious (secularophobia?), including fewer people who fear atheism (atheismophobia?).