What has medieval philosophy ever done for us? Seriously, name a thinker of merit to emerge from the 5th to 15th century. Thomas Aquinas? William of Ockham? Mere curiosities today, one might argue; part of an irrelevant tradition of religious superstition.

Prof Peter Adamson, creator of the History of Philosophy Without any Gaps podcast and book series, begs to differ.

“For starters, precisely because of their importance in the history of religion, medieval philosophers remain relevant in some cultures and contexts,” he says.

“If you want to understand the doctrines of the Catholic church you had better know your Aquinas, and in the Islamic world today people still have strong views – both positive and negative – about medieval thinkers such as Averroes and Avicenna. ”

Secondly, says Adamson, “you can’t understand where the ideas of famous figures of early modern philosophy such as Descartes, Spinoza and Leibniz came from without knowing about medieval philosophy”. Thirdly, “it’s just not true that medieval philosophy is always about topics in religion. They [the philosophers] address the full range of philosophical topics, from ethics and political philosophy to logic, philosophy of language, you name it.”

Of further interest today is the fact that some of the most significant thinkers of medieval times emerged from the Arab world in the Islamic “golden age” of the 8th-13th centuries. This was an era when Muslim thinkers were at the forefront of reasoned debate in mathematics, science and philosophy.

Adamson, a specialist in ancient and medieval philosophy, highlights in his latest book Philosophy in the Islamic World just how influential certain theologians and mystics from this milieu have been. Asked to single out one thinker, he names the Persian polymath Avicenna (980-1037) who invented “probably the most influential and interesting medieval attempt to show that God exists”.

Just how influential was he?

“In the Islamic world people who called themselves ‘philosophers’ at first responded above all to Aristotle,” Adamson explains. “But once Avicenna came along, doing philosophy meant responding to him.”

How did Avicenna ‘prove’ God exists?

“The full argument is a bit complicated, but here is a somewhat simplified version. Avicenna’s proof actually has nothing to do with design, he doesn’t need the idea that the universe is intelligently put together. Instead, he argues from the idea that the things we see around us are ‘contingent’ or merely ‘possible’.

“The idea here is that a contingent thing is something that may either exist or not exist; its nature does not guarantee that it exists. What Avicenna wants to do is show you that although all the things we experience directly are indeed contingent, there is also something else that exists necessarily, in other words, whose very nature guarantees that it exists.

“To do this, Avicenna points out that since a contingent thing on its own merit could either exist or not exist, it must have some external cause that made it exist - like ‘tipping the scales’ in favor of its existence rather than its non-existence.

“So take me, for instance. I am contingent, meaning that I am the sort of thing that could easily have failed to exist. In fact, at one time I didn’t yet exist, and in the future I will cease existing, that proves I’m not necessary.

“So there must have been a cause, maybe my parents, who brought me into existence. Now Avicenna observes that the aggregate whole of all contingent things – in other words the physical universe – is also contingent. After all, everything in the universe is contingent, so taken all together as one thing, it too must be contingent. Thus it also needs an external cause, just like I do.

“Since that external cause has to be outside the whole aggregate of contingent things, it cannot itself be contingent. So it is necessary. Hey presto, we’ve proven that there is a necessary existent which causes all other things! And this, of course, is God.”

How did this argument mark an advance on theological proofs in the Christian world?

“One thing I like about this proof is that it captures, in rigorous terms, a reason that I think actually underlies people’s belief in God. Effectively, Avicenna is trying to show that when you look around and think, ‘All of this could have failed to exist; why is there something, rather than nothing?’ you are asking a good question.

“The answer to the question is that not everything can be contingent; that is, not everything could have failed to exist. There must be something that just has to exist, to explain why everything else has wound up existing.

“This contrasts favourably to other medieval proofs, which turn on clever but unconvincing conceptual tricks like Anselm’s ontological argument, or do invoke the intelligent design of the universe, which many people nowadays think is a premise discredited by science.”

Philosophical debate in the Islamic world, as you depict it, seems to have been quite robust and at times fearless in previous centuries. Was there a relatively high degree of intellectual freedom then?

“There were certainly examples of religious and intellectual persecution in the pre-modern Islamic world. But it would be fair to say that these were not the norm and that, especially in the ‘classical’ or ‘medieval’ period of Islam, philosophical thought was far less constrained than in contemporary Latin Christendom.

“We shouldn’t be surprised by this, because in sunni Islam there is no hierarchical institution like the Western Church that could try to enforce orthodoxy. Rather, there was a class of scholars that have religious authority through their learning, but for the most part these people weren’t in a position to enforce whatever they took to be ‘correct belief’.”

As you continue with your project of compiling a history philosophy “without any gaps”, have you discovered any variation between cultures in the acceptance of women philosophers?

“I have covered four cultures so far in the podcast, and in the books based on them: the ancient Greek and Roman world; the Islamic world; ancient India (this I have been covering with a co-author, Jonardon Ganeri); and Latin medieval Christendom. Of these four, by far the richest tradition for women thinkers is, surprisingly, the last one.

“We have a whole series of medieval female authors whose works actually survive. The most famous is Hildegard of Bingen, but there are numerous other philosopher-mystics like Hadewijch, Mechthild of Magdeburg, Marguerite Porete - who by the way is an example of a thinker of medieval Europe who was put to death for her teachings - and Julian of Norwich. ”

“Another particularly fascinating figure is Christine de Pizan, who lived in the early Renaissance and is perhaps the first woman who wrote surviving works on a wide range of philosophical topics, including political philosophy.

“Back in antiquity, the situation was less favorable.

“As for Islam and India, I was disappointed to find that although there were female Muslim intellectuals - especially religious scholars - before the modern era, one is hard pressed to name any women philosophers in classical Islam beyond certain mystics, including an important early one named Rabia.

“Ancient India is a fascinating case. There are texts presenting us with wise women in debate with male philosophers, as in a couple of passages from the Upanishads. It seems this must depict a real phenomenon, though as with European antiquity we don’t have many, or perhaps any, surviving works that were actually written by women.”

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Ask a sage:

Question: The negative aspects of Twitter are starting to outweigh the positive. Should I stop using it?

Hildegard of Bingen replies: “We cannot live in a world that is not our own, in a world that is interpreted for us by others. An interpreted world is not a home. Part of the terror is to take back our own listening, to use our own voice, to see our own light.”