Nowadays, not many philosophers are prominent enough to get nicknames. In medieval times the practice was more popular. Every scholastic worth their salt had one: Bonaventure was the “seraphic doctor”, Aquinas the “angelic doctor”, Duns Scotus the “subtle doctor”, and so on. In the Islamic world, too, outstanding thinkers were honoured with such titles. Of these, none was more appropriate than al-shaykh al-raʾīs, which one might loosely translate as “the leading sage”. It was bestowed on Abū ʿAlī Ibn Sīnā (d.1037 AD), who was known to all those medieval scholastics by the Latinized name “Avicenna”. And not just known, but renowned. Avicenna is one of the few philosophers to have become a major influence on the development of a completely foreign philosophical culture. once his works were translated into Latin he became second only to Aristotle as an inspiration for thirteenth-century medieval philosophy, and (thanks to his definitive medical summary the Canon, in Arabic Qānūn) second only to Galen as a source for medical knowledge in Europe.
In the Islamic world, Avicenna’s influence was even greater. Here he effectively replaced Aristotle as the central authority for philosophy. Even the term “Peripatetic”, which originally meant “Aristotelian”, started to mean “Avicennan” instead. Critics and admirers of Avicenna agreed that his thought was all but equivalent to philosophy (falsafa) itself. To criticize the “philosophers” as did al-Ghazālī in his famous Incoherence of the Philosophers (Tahāfut al-falāsifa), or as did al-Shahrastānī in his much less famous but more entertainingly titled Wrestling Match with the Philosophers, was to enumerate the errors of Avicenna, not those of Plato or Aristotle.
Apart from the emphatic criticism, this is pretty much what Avicenna had in mind. He deliberately set out to supplant Aristotle by re-organizing and re-thinking “Peripatetic” philosophy. For this reason he did not write commentaries on Aristotle, like others did (for example al-Fārābī and Ibn Rushd, known in Latin as “Averroes”), but instead produced original treatises with varying lengths and levels of difficulty. Even in his most Aristotelian compendium of philosophy, the one that would prove most popular in Latin translation, Avicenna departed from Aristotle’s teaching whenever he saw fit. This was the Shifāʾ (Healing), a many-volume text with sections on every area of philosophy including mathematics and physics. Avicenna wrote other overviews of his own thought, such as the Najāt (Salvation), (more or less a shorter version of the Shifāʾ), and al-Ishārāt wa-l-tanbīhāt (Pointers and Reminders), still lengthy but much more compressed, designed to make the student reader work to reconstruct the line of thought behind the teachings.
What were those teachings, and why did Avicenna think it was worth departing from the authority of Aristotle to devise them? There is no simple answer, since Avicenna came up with innovative and influential ideas in so many areas. For instance his system of logic, while still broadly within Aristotle’s paradigm, made new proposals on topics such as modality (the concepts of impossibility and possibility) and the truth conditions of propositions (for instance, the question of whether the subject of a proposition must exist right now for the proposition to be true). Right down to the modern era, works on logic written in the Islamic world, from West Africa to Islamic India, worked creatively within the Avicennan paradigm the way that Avicenna had worked creatively within Aristotle’s. on the closely related topic of knowledge, Avicenna’s epistemology was just as revolutionary. His much-misunderstood idea of “intuition” (ḥads), intended to explain how geniuses like himself were able to devise syllogistic arguments, was pressed into the service of mysticism by some later authors. A less famous but more fundamental feature of his epistemology is his forthright essentialism, according to which to grasp a thing’s nature involves seeing all the intrinsic, constitutive features that belong to that thing’s essence.
One application of this idea (or so I’ve argued in a paper I recently wrote with Fedor Benevich) came in his famous “flying man” argument. This is a thought experiment in which we are asked to imagine a mature human created in mid-air by God. The human’s sight is veiled and there is no sound, smell, etc. Thus the newly created person is experiencing nothing at all through his senses, and has indeed never enjoyed any sense-experience (he was only just created). He doesn’t even know that he has a body. Avicenna argues that this person will nonetheless be able to grasp his own existence. What this shows, for him, is that the body has nothing to do with what is essential to human beings. If it did, it would be impossible for the flying man to grasp himself, just as it is impossible to grasp the nature of humanity without grasping that humans are essentially living things.
Later thinkers appreciated Avicenna’s idea that we all have an intimate, direct grasp of ourselves. Centuries later in Safavid Persia one admirer, the Iranian philosopher Mullā Ṣadrā, asserted that even a non-human animal would be able to grasp itself if created in the same situation. More generally, existence (wujūd) was at the center of Ṣadrā’s teaching, and this too was ultimately an inheritance from Avicenna. In another domain where he was enormously influential, Avicenna had reconceived Aristotelian metaphysics as a study of existence. Avicenna distinguished between different types of existence: it is one thing to exist in the real world, another to exist only in the mind; one thing to exist contingently, another to exist necessarily. Both of these contrasts become pervasive in all post-Avicennan writing on metaphysics.
Avicenna made existence central to his proof of God’s existence. Even this argument had its own nickname, burhān al-ṣiddiqīn, the “demonstration of the truthful”. It begins with the idea that something can exist either contingently or necessarily. To exist “contingently” means to be capable either of existing or not existing. Look around you: everything you see is like this, something that does exist but might not have done so. To exist “necessarily” is to exist in a way that rules out non-existence. In other words, it is to have guaranteed existence by its nature. (Conversely an impossible thing like a round square would be guaranteed not to exist.) The question at the centre of Avicenna’s proof is, then, whether anything exists in this necessary way. He wants to say that there is indeed a necessary existent, but only one of them, and that this is God.
Avicenna thinks he can get to this conclusion by considering contingent things (that is, anything at all that can exist, from dogs to the Eiffel Tower, except for God himself). Contingent things are those that might exist or might not. In themselves, they do not “deserve” to exist, and so if they do, it must be because something else has come along and made them exist rather than not existing. Actually this is another application of Avicenna’s essentialist epistemology: we can see that it is no part of the essence of any contingent thing that it must exist. As it’s often put in the Avicennan tradition, a contingent thing needs to be “preponderated” to exist or not exist, like “tipping the scales” in favour of one option or the other. What tips the scales in favour of a thing’s existing will be its external cause: for the Eiffel Tower this was Gustave Eiffel, along with a lot of labourers, for the dog it is the dog’s parents, and so on. Now, consider not just one contingent thing, like the Eiffel Tower, but all the contingent things that exist taken together as a totality (effectively, this is the universe, past present and future). Ask yourself whether this totality is contingent. Obviously so: just as a totality of water will be wet, so a totality of contingent things will be contingent. By the same reasoning then, this totality too needs an external cause to make it exist. And since that cause is external to the totality of contingent things, it cannot itself be contingent; it must be necessary. And this is God.
Avicenna’s proof left his successors in a quandary. on the one hand they loved the proof; and what’s not to love? Not only is it ingenious, it also captures in rigorous form an intuition that in fact moves many theists to believe in God: that there must be a creator, since otherwise we cannot explain the sheer existence of the universe. In this respect his burhān al-ṣiddiqīn outdoes some other famous proofs of God, like Anselm’s ontological argument, which is so far from capturing widely held intuitions that even devout theists usually think there must be something wrong with it. on the other hand, later thinkers were concerned with Avicenna’s close association between God and necessity. Avicenna assumed that God must be necessary in every respect, free of all contingency. Otherwise, by the logic of his argument, some cause would be needed to “preponderate” God to be the way that He is, when He might have been otherwise.
In particular, Avicenna held that God must be eternally causing the universe to exist. If He went from not creating the universe to creating it, then this could only be explained by an external cause that preponderates God to begin creating. For al-Ghazālī and other theologian critics of Avicenna, this conclusion was unacceptable. They argued instead that God can simply will to create, after not creating. He is, so to speak, like a set of scales that can tip itself. But this simple response raises further problems. For one thing it seems to undercut Avicenna’s proof for God’s existence, which requires the premiss that all contingencies need an external cause, that all scales need something else to tip them. What’s more, the notion that God’s will simply asserts itself at some point, apparently after waiting for an eternal amount of time before making its move, makes creation look like an arbitrary, even irrational event.
The issues at stake here resonated throughout later philosophy among Muslims, and among adherents of other religions too. Jewish medieval philosophers engaged with Avicenna, albeit mostly indirectly, and as mentioned the Latin scholastics read him avidly and referred to him constantly. The question of whether God indeed has an irreducible will, one that arbitrarily selects one possible outcome when options are available, became central in medieval Christendom with voluntarists like Scotus and Ockham. These developments, and later ones such as the use of modality in the metaphysics of Leibniz, were no simple retreads of what we find already in Avicenna. But Avicenna was always in the background, either as a direct source or as a significant if hidden presence lurking behind key debates of early modern philosophy.
As for philosophy in the Islamic world, centuries’ worth of thinkers went on to engage with the Avicennan ideas sketched above, and many more besides. This is clear even from the number of commentaries written on his treatises, especially on the dense and difficult Pointers and Reminders. A giant of twelfth-century thought like Fakhr al-Dīn al-Rāzī engaged with Avicenna on practically every page of his own encyclopedic theological-philosophical treatises, and he went on to be a touchstone for later figures. If you have heard of Avicenna and Averroes but not Fakhr al-Dīn and are wondering why, the answer is simple. The medieval European translation movement did not render his works into Latin. All those “doctors” like Bonaventure, Aquinas and Scotus, who made so much use of Avicenna, were completely unaware of Fakhr al-Dīn. Until recently, scholarship in European languages largely followed suit, with most research being directed only towards figures from the Islamic world who had an impact on Latin medieval and Renaissance thought. This created a widespread myth that there was a decline or even demise of philosophy in the Islamic world: in fact there was an explosion of activity with hundreds of treatises written, expressing every conceivable viewpoint along a spectrum from an enthusiastic embrace of Avicenna to cautious modification to outright condemnation. Now, as more research is finally being conducted on the many thinkers whose works were not received into Europe, it is becoming even clearer that from the eleventh century onwards, philosophy in Latin Christendom and in the Islamic world was to no small extent a matter of writing footnotes to Avicenna.
Peter Adamson is Professor of Late Ancient and Arabic Philosophy at the Ludwig-Maximilians-Universität in Munich, and author of the book series A History of Philosophy Without Any Gaps