There are many theories about the causes of the decline of Islamic philosophy and science. For example, both David C. Lindberg in his book The Beginnings of Western Science and Hillel Ofek in his article Why the Arabic world turned away from science mention political upheavals and the gradual fragmentation of the Abbasid caliphate as possible factors. One traumatic event often mentioned is the devastation of Baghdad, the center of Islamic scientific activity, caused by the Mongol invasion in 1258.
None of these theories sounds convincing. Though the Mongols destroyed the Baghdad library—the largest library in the world at that time—, many of the books were available in copies in other places not touched by the Mongol invasion, for example in Islamic Spain. Also, as the chart above indicates, the decline started already up to 200 years before the Mongol invasion. The chart also shows that there was in fact a recovery in the thirteenth century—the century when the Mongol invasion happened. This has to do with the establishment of the Maragha observatory in northeast Iran in 1259, supported by the Mongol conqueror Hulagu Khan, just one year after he sacked Baghdad.
Furthermore, though the Mongol invasion has affected science severely in Baghdad and in other parts of the Islamic domain, history shows that countries and even cultures can quickly recover from devastating but relatively short-duration events. As an example, the Black Death epidemic in the middle of the 14th century killed 30-60% of Europe’s population. This devastation didn’t stop scientific inquiry or any other creative activity in Europe. On the contrary, the flourishing of European Renaissance followed shortly thereafter. Thus, it seems that the destruction of Baghdad by the Mongols doesn’t explain the decline of scientific inquiry in Islam.
Political upheavals abounded in Europe between the 14th and 17th centuries, including many wars—some of them long lasting like the 100 Years War between England and France during the 14th and 15th centuries and the devastating 30 Years War in Germany in the 17th century. These upheavals didn’t halt the steady progress of science in Europe. There is no reason to think that political upheavals would have had a different effect in the Islamic world. The same can be said about the gradual “fragmentation” of the Abbasid empire: the Christian world was also very fragmented, but this didn’t lead to the decline of science there. In fact, the Reformation movements in the 16th century—causing large divisions in European society—just preceded the Scientific Revolution.
Finally, relative political stability soon returned in the Islamic world after the demise of the Abbasid caliphate—at the latest when the Ottoman empire was established. This, however, didn’t result in a recovery of scientific productivity.
An altogether different—and in my view more fruitful—attempt to explain the decline of Islamic philosophy and science emphasizes the importance of philosophical foundations for science and the threat that religion poses for those philosophical foundations.