ONE OF THE MOST common prejudices we historians of philosophy encounter is the notion that philosophy is somehow incompatible with religious belief. Religion is based on faith, philosophy on reason; religion is rigorously imposed doctrine, philosophy is open-ended inquiry; religion is about believing what you’re told, philosophy about figuring things out for yourself. A moment’s reflection will show you that it must be a little more complicated than that. After all, nearly all philosophers in history — famous and obscure, ancient and modern, Western and non-Western, male and female — have been religious believers. No surprise there, given that nearly all humans in recorded history have been religious believers. So to believe in a fundamental opposition between religion and philosophy, or faith and reason, is to assume that nearly the entire history of philosophy has consisted of people rising above or setting aside their own deeply held spiritual convictions.

In fact, the reverse is true. Religion has always been a significant factor in shaping philosophical development and progress. It is only one such factor, of course. Christian theology shaped Aquinas’s metaphysics in much the same way that, say, ideas about family relations in ancient China shaped Confucius’s ethics, or the effects of the Industrial Revolution shaped Marx’s economic theory. Furthermore, many thinkers have reflected on the relationship between religious authority and philosophical reasoning. Aquinas himself is a famous example because of his attempt to explain how revealed truth supplements the deliverances of natural reason. But the question has arisen in other cultures too. Just take the ancient Indian Mimamsa school, which developed an entire epistemology and philosophy of language to buttress their claim that the Vedas are an authoritative source of knowledge. Or take the 17th-century Ethiopian philosopher Zera Yacob, who tested his religious beliefs to discover whether they would hold up under rational scrutiny.

In short, then, we should see religion not as incompatible with philosophy, but to the contrary, as providing a context in which philosophical reflection has typically been undertaken. One can say this while wondering whether certain religious ideologies, though not religion in general, have inhibited or altogether suppressed the urge to philosophize. One of the best examples comes from the cultural context that produced the aforementioned Aquinas. Around the time of his death, two rounds of condemnations were issued in an attempt to rein in the freewheeling explorations being conducted at Aquinas’s home institution, the University of Paris (he was almost certainly one of the targets). Then again, the fact that Aquinas, and after him brilliant and influential philosophers like Duns Scotus, worked in this very time and place shows just how intractable the censor’s task is. Good philosophers are, apparently, hard to keep down. Indeed, some have plausibly proposed that these and other condemnations of philosophy have unintentionally given rise to ingenuity and innovation.

Another cultural context often imagined to have restricted the free exercise of reason is one that is far larger in its chronological and geographical span: the whole religion of Islam. Again, a moment’s brief reflection should show how preposterous this is. Islam arose in the seventh century and quickly spread over a territory that dwarfed Western Europe; today the Islamic world stretches even further than it did then, with Muslim populations dominating countries from Malaysia and Indonesia in the East to Morocco in the West. Is it really credible that there has been a uniform attitude toward philosophy, or reason in general, across one and a half millennia of Islamic history and cultures strewn over half the globe? Obviously not, yet one frequently hears that Islam in its very essence is somehow opposed to rational reflection, is to blame for the failure of all these cultures to experience the Enlightenment (as if the Enlightenment happened in Europe just by default, so that its failure to occur elsewhere needs explanation), and so on.

One way to rebut such charges is to sift through the historical record and produce examples of rationalism within Islamic culture. This is the project undertaken by Souleymane Bachir Diagne in Open to Reason: Muslim Philosophers in Conversation with the Western Tradition, a monograph first published in French in 2008 and now translated into English by Jonathan Adjemian. Like most general publications on philosophy in the Islamic world, he focuses mostly on the “medieval” period — in the Islamic context, one might say “classical” or “formative” — roughly the ninth to 12th centuries CE. Insofar as his task is simply to offer us examples of rationalist thinkers, it is an easy one. For as it turns out, Muslims and also Jews and Christians living in the “medieval” Islamic world were much more given to rationalist philosophy and much freer to explore ideas wherever they might lead than were their contemporaries in Christian Europe. Both in the Latin West and Byzantium, sophisticated and valuable philosophy was certainly done, but condemnations were a potential threat and occasional reality. By contrast, and again in stark opposition to what has somehow become popular belief, there was no systematic political suppression of philosophers in the medieval Islamic world.

Diagne moves through what will, for those who know something about the field, be a fairly familiar cast of characters. We hear about the Greek-Arabic translators, disquiet with the fruits of their work in the shape of an attack on Greek logic from a grammarian named al-Sirafi, the towering achievement of Avicenna, al-Ghazali’s critical response to Avicenna, and so on. The most rationalist of the rationalist philosophers (and also a Muslim jurist — so both very religious and very rationalist), the 12th-century Andalusian commentator Averroes, receives due attention for his bold claim that philosophy is obligatory for any Muslim who is capable of this form of intellectual work. All of this is deftly sketched, but ultimately very familiar from other introductions to the topic. Admittedly, some of Diagne’s choices are more surprising. For example, his treatment of Avicenna revolves around a very unusual work from the latter’s corpus, which, as Diagne admits, is “not one of his major works,” and in fact of disputed authenticity. Entitled The Book of Ascent, the text offers an allegorical reading of a story about the Prophet Muhammad’s visitation from an angel and “night journey” to Jerusalem. Given Diagne’s interest in the relation between philosophy and religious discourse, it makes sense for him to focus on this work. But one does come away from the chapter without any sense of why Avicenna became the towering figure of this whole philosophical tradition.

A weakness of the book is something I have already mentioned, namely that its focus is restricted to the first few centuries of philosophy in the Islamic world. After a glancing reference to a couple of post-classical thinkers (Suhrawardi and Mulla Sadra), Diagne leaps ahead by about 700 years to discuss more recent philosophers like Jamal al-Din al-Afghani, Muhammad Abduh, Muhammad Iqbal, and Ali Abdel Razek. These chapters are very welcome, since modern figures are usually left out of histories of philosophy in the Islamic world. But I wonder whether Diagne would approach the book this way were he writing it now, with the same yawning historical gap at its center. Even in the decade since the original French version was published, there has been an explosion of scholarly interest in later philosophy written in Arabic and Persian. This work has shown that philosophical concepts and arguments, especially taken from Avicenna, became a standard part of the intellectual equipment of theologians and mystics from North Africa all the way to Central Asia. In fact, to mention a tradition which Diagne knows well but does not discuss in this book, philosophical theology and mysticism also penetrated into West Africa and was practiced at places like Timbuktu. The thousands of manuscripts from this culture have hardly been studied, but even their titles give us a glimpse of the vibrant intellectual culture there. The same can be said about the Ottoman Empire or about India during its period of Islamic rule.

We know, then, that generally speaking Islam has no more precluded the pursuit of philosophy than Christianity did in Europe or, for that matter, than paganism did in ancient Greece. What we don’t yet know is all the details. As I’ve said, we need to realize that philosophy in the Islamic world has never been just one thing, any more than Islamic culture has been just one thing. It involved Aristotelians and theologians, mystics and scientists, and Jews and Christians as well as Muslims. Diagne ends with an impassioned plea that resonates well with this observation. Following Abduh, he emphasizes that Islam itself has changed with the world around it, and argues that Muslims should be open to this change. Denouncing the fundamentalists who have a “horror of time,” and want to return to the set of beliefs and practices they claim to find among the earliest generations of Islam, Diagne encourages believers to “adapt […] tradition to the changes that are arriving” as history moves forward. They should, in other words, do what philosophers too have always done: think new thoughts within the context into which they were born.

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Peter Adamson is professor of Late Ancient and Arabic Philosophy at the LMU in Munich and produces the podcast A History of Philosophy Without Any Gaps.