Finally the answer I promised is ready. Excuse the long delay. I had little free time to compose this and needed to brush up on some sources for greater accuracy and inclusiveness. Although not too unexpected, the outcome turned out quite like an introductory essay. So I also suspect this might not make for an appropriate post or Q&A for the main site in which case I may propose this instead posted in the site blog perhaps after some modifications in structure and details. But I found it worthwhile to write this extensive answer since this can serve to familiarize people with the intellectual background of some of my own occasional philosophical answers in this site (that are sometimes perceived to be "eccentric" by some users), and also because this kind of questions would be common with anyone seeking something like a more objective approach to evaluating religions’ truth claims, an attitude that can be fortunately richly addressed by Islamic intellectual legacy that sadly however remains largely marginalized and inaccessible to this day because of the narrow methodological attitude that has been reigning over the mainstream scholasticism in the Muslim world.
Historically, this narrowness is rooted in a background of scholastic skepticism and pessimism towards intellectual approaches to Islamic study that had emerged among more “open-minded” (yet often equally as pious and faithful) Muslim thinkers due partly to their introduction to Greek philosophies as well as inspiration found in more esoteric aspects of Islamic scriptures.
The opposition by mainstream scholastics was directed against both the discursive method of Greek philosophers adopted by Muslim philosophers as well as the interpretative doctrines of the Sufis or Muslim mystics who attributed those doctrines to inner realization of transcendent truths to which the literal texts make only verbal references and remain veiled in literal readings.
Ahmad ibn Hanbal, and al-Ghazali have been the two most prominent figures in this skeptical camp that I guess you're fairly familiar with. Although part of the objections by such scholars to the practice of reason, philosophy and theoretical mysticism by Muslim philosophers and mystical interpreters were legitimate (but not that their own alternative views were more coherent), but their works were nonetheless influential in establishing a later persisting disinterest in intellectual approaches to religion in the Sunni Arab world.
This background of negative attitude towards philosophical approaches to Islam over the last century in particular has been further reinforced by the widespread of the Salafi movement in Islamic world, a so-called “reform” movement which in its quest for restoring “pure” Islam advocates an uncritical literalism and strongly opposes attempts at rational understanding of religion (What I wrote so far must be familiar with most fairly educated Sunni/Arab Muslims but what follows is definitely news!)
However this was not how things unfolded in the eastern part of the Islamic world especially in Persia. Here, interest in various forms of philosophy and esoteric understanding of religion continued and flourished with evolutionary outcomes that not only overcame the defects and criticisms concerning the earlier generation of philosophers and mystics but also capable of arriving at explanations of religious scriptures that were both profound and consistent with more esoteric parts of scriptures and at the same time faithful to the apparent meanings of the literal text.
For a more detailed picture of this cultural divergence and an intro to the latter, have a look at this and this short essays from the Routledge Encyclopedia of Philosophy. For an extensive introduction you will have to read works by scholars such as Henry Corbin and Seyyed Hossein Nasr on history of Islamic Philosophy.
My following perspective to your questions is informed by this latter legacy of Islamic intellectual heritage which today find recognition mainly in Iranian academic and seminary circles and has only recently attracted substantial interest from Western Academia.
For laying out my full answer, I have to rather reformulate your questions of a priori proofs for Islam into two broad sets. This gives us questions concerning
- proofs for validity of Islam’s basic doctrines; and
- proofs for validity of Islam’s particular doctrines.
This is an important distinction because basic doctrines of Islam such as God, soul, angels, supernatural worlds and afterlife are broadly shared by most major world religions (even though theologians from each religion and within themselves inside each religion often disagree significantly on the details and extended definition of these doctrines). So proofs for these basic doctrines works to give a general credence not just to Islam but more or less to several Abrahamic and to some extent even non-Abrahamic religions. As a side note on this observation, I find it worthwhile mentioning a relevant view by the adherents of the school of Perennial philosophy who, based on the concepts of Neo-Platonic philosophy, argue that these substantial commonalities between world major religions are indicative of a universal transcendent truth and origin that underpin them all, and that differences among them result only from the temporal conditions in which the universal religion has been revealed.
This is an interesting view that happens to be on the general level in conformity with Islamic belief in common Divine origin of all Abrahamic religions, and a view that is important in comparative study of religions since it indicates that despite conflict on the more outward and temporal manifestations of world major religions, they are not necessarily mutually exclusive on deeper levels, and that there are religions or scholars within these religions that endorse this view.
But as concerns Islam, you will find that in this shared domain, different brands of Muslim intellectuals in particular have historically contributed to the development of a very rich and colorful legacy of explanatory thought and intellectual interpretation of Islamic basic doctrines that embodies a wide range of a priori proofs and arguments that substantiate and elucidate the extensive nature of several of the aforementioned primary truth claims by religions. In the next part, after a broad introduction to primary methodological approaches advocated by different brands of muslim thinkers, I will introduce some of the most prominent of these thinkers and their intellectual achievements.
Methodology-based classification of Muslim thinkers
From the works of Muslim thinkers in general different methods and approaches can be found to justification or demonstration of primary doctrines of religion. These thinkers can be mainly classified into three groups.
Theologians or practitioners of Kalam, who are divided into several schools, rely on dialectic reasoning to substantiate established religious doctrines or resolve controversial or ambiguous parts of the scripture.
The earliest prominent theological schools are three:
Among historical subjects of contention among these three schools (excluding the apparently more political Shia-Sunni controversy on Prophet’s succession but nonetheless one with deep theological foundations and implications) have been the questions of human free will vs Divine predestination, the role of human reason in explaining religion, and (un)createdness of Quran.
Throughout history, theologians who recognized human free or religion-aided reason as a legitimate source for explaining religious doctrines, have attempted to come up with rational arguments for supporting religious beliefs. The most prominent proof that has been adopted and presented by these thinkers is the Teleological Argument for God’s existence.
Muslim Philosophers have historically succeeded in formulating independent philosophical proofs for several religious doctrines as part of their general interest in Metaphysics. This group of thinkers distinguish themselves from Theologians as they claim that throughout their philosophical activity they are not biased by any presupposition or preconceived belief be it religious or otherwise, but they solely follow the direction to which reason and logical thinking alone take them even though many prominent figures among them have been of the opinion that true belief and reason do not conflict and that reason in its full fruition confirms what has been revealed by religion. Muslim philosophers, to ensure objective ground for their philosophies, much like the modern analytic philosophers, proceed primarily from self-evident principles and, secondly, intuitive and empirical premises to derive new truth statements through deductive reasoning.
Finally Sufis or Muslim mystics have their own way of “realizing” truths as claimed by religion through spiritual exercise and self-purification. For them the most intimate knowledge of the sacred truths is obtained not through the conceptual mind but the heart which is the place where extra-mental realities are realized through existential unity. Philosophers in their eyes can at best obtain a vague mental image of spiritual truths while a Sufi bears the objects of truth in his heart. Moreover, since emergence of Ibn Arabi, who is attributed with the establishment of a discipline named "Theoretical Mysticism" among Iranian scholars, masters of this approach have engaged in practicing logical reasoning in order to make intuitively-realized truths accessible to other thinkers on the conceptual level.
It is important to note that the interest of Muslim thinkers in both rational and intuitive modes of approaching religion has been generally inspired by the Islamic scriptures themselves. Statements from Quran and Hadith as to the importance of intellection in affirmation of religious truths, as well as verses that imply man’s ability to observe transcendent realities have been invoked by these two groups of Muslim scholars in support of their respective epistemological/methodological approaches to religion and their favored interpretations.
These three intellectual traditions within the Islamic world have gone through a steady process of expansion, criticism and evolution with thinkers commenting on or criticizing the methods and theories presented by other thinkers both within and outside their own traditions.
In the next part of my answer I try to provide a very broad summary of some of the prominent faces of philosophical and mystical thinking in the Islamic world and specific demonstrations of religious doctrine that they have presented in their philosophies. My summary excludes theologians for I guess they are better known with the mainstream as well as the fact that the theories that come from dialectical reasoning of theologians are often debatable and therefore indefinite in their conclusions (with the exception of more philosophized Imami theology as formulated by Khaji Nasir ud-Din al-Tusi) compared to those that come from non-biased a priori approach of the philosophers and the synthetic (i.e. demonstrated intuitions) approach of the Sufi theorists.
Prominent figures with remarkable demonstrations of doctrines
Note that this list is a selection of the most original and influential figures in shaping of Islamic intellectual legacy from among a few dozen influential Muslim philosophers and mystics that have emerged throughout history.
Al-Farabi who is credited with the foundation of the Islamic tradition of philosophy other than attempts at reconciling the philosophies of Plato and Aristotle, he introduced his own innovations in logic and philosophy. Among his notable contributions is his treatment of Divine revelation and Prophethood as part of the discipline of Metaphysics. Farabi believed that religion and philosophy talk about identical truths but only through different language for different audience.
In his philosophical approach to religion, Farabi equated the idea of Active Intellect posited by the Greek philosophers as the source of philosophical enlightenment with the idea of Angel of Revelation in religion. He argued that Prophets are able to find enlightenment from God and the transcendent world due to their strong faculty of imagination which enables them to visit Gabriel in the astral plane where they receive Divine knowledge therefrom in the form of imagery and metaphor. Farabi also discussed the existence of human immaterial soul (nafs) and its different faculties.
Avicenna, who was an incredible polymath genius, among diverse contributions to science, expanded on the works of Farabi. Among his major contributions in this field, he expanded the science of soul as formulated by his predecessor, and formulated the first Ontological argument for demonstrating the existence of God as "the Necessary Being."
Muh’y id-Din al-Arabi, was by far the most prominent master of practical and theoretical mysticism or Sufism in the Islamic world who is renowned for his account of relation between God and creation on the basis of the theory of Unity of Existence, as well as his theory of Perfect Human as the fullest manifestation of the Divine that corresponds to the primordial light of Prophet Muhammad.
The emergence of 12th century Suhrawardi, the founder of the so-called Philosophy of Illumination in the Islamic world, was the first major move towards convergence between the analytic and discursive method of Muslim Peripatetic Philosophers to religion on one hand and the intuitive and spiritual approach of the Mystics or Sufis, on the other, even though he considered spiritual realization of the Mystics superior to the conceptual comprehension of philosophers based on the classification of knowledge into “knowledge by presence” and “knowledge by acquisition” which has been the epistemological bedrock for all major Islamic philosophical schools. Suhrawardi also explained God and creation in terms of a very profound symbolism of light and darkness which along with his philosophical account of the astral plane and angelology was another major development towards convergence of philosophy and religious doctrine in the Islamic world. Surawardi is also known as the first philosopher to extensively quote Quran and Hadiths as scriptural testimonies to philosophical theories that he argued through independent reasoning.
The last thinker who is probably the most noteworthy is Mulla Sadra, the Persian Shia Metaphysician and Mystic of the 17th century, renowned for fulfilling the aspiration of Suhrawardi by achieving a grand synthesis and reconciliation of the major thinking traditions in Islamic and world history (i.e. literalist, theological, Peripatetic, Illuminationist and Mystic/Sufi) to form a brand new philosophical school that he termed Transcendent Theosophy.
Among his precious contributions to theist philosophical thought has been his innovative proof and ontological explanation of the Illuminationist account of Divine essence, attributes and acts based on the theory of Principality of Existence over Quiddity which forms the bedrock of his philosophy, his proof for the immaterial nature of human imagination, his theory of Substantial Transformation (or Motion) which explained how corporeal and incorporeal forms go through constant evolutionary change, and his theory of unity between the knower and the known, which together enabled him to form a substantial account of the doctrine of human posthumous Resurrection by consequence. Mulla Sadra’s philosophy therefore represents the pinnacle of the attempts by philosophers and mystics for unmasking the intellectual foundation of religious doctrines via the objective method of demonstration.
This was a succinct summary of important icons in Islamic intellectual thought whose legacy embodies a priori demonstrations for several key religious doctrines (as well as ones that discredit the particular doctrines of other religions that conflict with Islam such as the Christian Trinity and the Indian Reincarnation which I will get to in a moment). The formation of this rich legacy is a reaffirmation of the view by Muslim philosophers that religion and reason are mutually confirmative, and this is not just by accident but, as proven independently by themselves and as corroborated by esoteric wisdom found in Islamic sources (examples in a moment), because religion and human reason have an identical source of inspiration which is Archangel Gabriel or the Universal Intellect. In giving credit to this historical achievement, Islam itself as a religion has the original claim, for formation of this legacy has been impossible without the inspiration that these thinkers found in the more esoteric aspects of the Islamic scriptures especially the hadiths by Shia Imams who are regarded as bearers of the esoteric knowledge of religion. I find worth concluding this segment of the answer by quoting a few hadiths from Imam Jafar as-Sadiq, the sixth Shia Imam and a renowned scholar for Sunnis, which corroborate this lofty view of intellect and its role in man's enlightenment and perfection in esoteric religion:
Indeed, Allah created Intellect — and it was the first creature from amongst the spiritual beings — from the right side of the Throne from His light.
Hisham (one of his close disciples)! Allah has placed two authorities over man: the apparent authority and the inner authority. The prophets and messengers are the apparent authorities and Intellect is the inner authority… O Hisham, the Commander of the Believers [i.e. Ali ibn abi Talib] has said, “Allah has not been worshiped by any means better than Intellect.”
The support of mankind is Intellect. From Intellect come sagacity, understanding, preservation, and knowledge and by using Intellect he gains perfection; and It is his guide, his instructor and key to his affairs. When his intellect is supported with Light, he becomes a scholar, a preserver, a reminder and a man of understanding. Through Intellect he learns the answer to how, why and where or when. He learns who helps and who harms him. When he learns this, he learns the routes, the connections and the differentiating factors. He then establishes pure faith in the oneness of Allah and acknowledges the need to obey Him. When he does so he finds the proper remedy for what he has lost and the right approach to whatever may come in. He knows well his present involvement, for what reason is he here, wherefrom has he come and to what end is he going. All these are due to aid of Intellect. (From Usul Al-Kafi by Muhammad ibn Ya'qub al-Kulayni, the Book of Intellect and Ignorance)
Islam’s comparative merit
In the above introduction of the legacy in which proofs for Islamic basic doctrines are found, reasons for Islam’s superiority as a particular religion can also be drawn. The fact that Islam’s message had the potential to inspire so many scholars and saints with profound explanatory thoughts about religion could be seen as a testimony to its intellectual excellence.
Additionally, this legacy also embodies philosophical proofs for those Islamic doctrines that conflict with those of other religions, such the Christian Trinity and the Indian Reincarnation. There have been Muslim philosophers who have put forward actual refutations for these said doctrines using the same method of logical reasoning.
Apart from the ontological doctrines that can be subject to philosophical and scientific examination, a substantial part of the task of comparing religions inevitably involves the examination of the outward manifestations such as scriptures and history.
On this level, the complications associated with interpretation, textual criticism and historical method arise and given the unique history of each religion, working out a predetermined universal criterion or formula to verify truthfulness of religions with regards to their worldly manifestations must be a daunting task and I don't know of any such attempt by Muslim scholars ever. But given my familiarity with the Muslim philosopher-scholars' high regard for methodological objectivity and their belief in supremacy of independent reason in establishing articles of faith before belief as well as the universalist wisdom of the Perennial Philosophy, a comparison framework that can be universally applied to the historical study of religions for evaluating their relative strength and/or truthfulness with respect to their historical performance, will be consistent with the methodology of this distinguished group of thinkers, although not strictly passing the kind of "orthodox" qualification that you had intended in your question.
Such a framework could be made of the following criteria:
- the degree of verifiability of scripture and history of each religion i.e. how much we can tell actual history from mythology.
- the level of integrity and virtue claimed for the central figures of each religion i.e. prophets, messengers and disciples;
- the consistency between doctrine and history, i.e. how consistent the claims of virtue by religions (especially for their founders) are with their actual historical performance;
- indicators of intellectual quality such as doctrinal consistency, rationality, richness and benefits of the revealed doctrine, ethics and law; which in turn can influence:
- positive/negative contributions to human culture and civilization by each religion.
It is needless to say some if not all of these criteria are closely interrelated. For example, when founders of each religion are to be compared for their relative merit, the verifiability and/or viability of the hagiographical accounts written by their followers is an important factor in the result of assessment, as well as the consistency between virtues stated for them in particular by the Divine scriptures and the actual recorded performance of those figures.
Here’s a random example to make this framework tangible: consider how Quran describes Prophet Muhammad “a mercy upon the worlds.” In the light of parameters of the above framework, we can credit Islam for claiming to have a Prophet who was not just merciful to his people but a mercy to all beings with respect to parameter #2. But a more fundamental question could be: “But was Muhammad really merciful at all? Didn’t he spread Islam by sword, after all?” to use a common Orientalist objection that falls within parameter #3; and finally regardless of the controversy over the wars “are all those accounts of mercy and kindness lavished on the Prophet true at all?” under parameter #1.
For an example to see what an argument within this framework especially with regards to the second and third criterion may look like you can have a look at this past Q&A which has been inspired by a similar kind of quest for a priori validation: Why take Muhammad’s words?. There you will see that by a close attention to the circumstances and character of Prophet Muhammad (as reflected in the scriptures and works of history) and contrasting them against man’s conventional wisdom I tried to argue for truthfulness of Prophet Muhammad in his claims. But to avert a natural objection to the unreasonable assumption of veracity of Muslim accounts of Prophet’s life, it is noteworthy that the above line of argument is based on mainstream historical narrative that is accepted by both Muslim and non-Muslim scholars as an accurate portrayal of Prophet’s character.
The above example however may be contested especially given a handful of prevalent misconceptions about Prophet’s conduct namely his wars and his multiple marriages but since it is off-topic to concentrate on that, I only suffice to say that there’s a good consensus that these negative judgments mainly result from a superficial understanding and not based on close and in-depth knowledge of Islam and Prophet Muhammad that can reveal the innocent nature of his wars and the noble reasons for his multiple marriages For this see: How did Islam Spread? By Sword or by Conversion?, Wikipedia: Spread of Islam, Marriages of the Holy Prophet and Wikipedia: Muhammad’s wives.
In regards with the first criterion it can be argued that Islam is the best documented religion in the world, considering that 1) its primary Holy Scripture has be written down under the supervision of the Prophet of Islam Himself by both his close and general companions with only compilation of the book taking place after his death and even that with the purpose of establishing only a dialect uniformity among the copies, and 2) early works of tradition and biography written about Prophet Muhammad reveal vast details about his conduct and life that allows for a much closer scrutiny of Islam as a religion compared to any other religion.
This feature, of course, is not in and of itself an evidence of Islam’s superiority or truthfulness, but one that gives Islam a degree of detailed and extended verifiability that other religions cannot even remotely match. This extensive verifiability is specifically significant with regards to those Islamic doctrines whose demonstrations depend on rigorous historical verification, such the belief in authentic preservation of the Holy Quran as well as its Divine origin when for example argued from the Holy Prophet's historically verifiable unletteredness, the gradual, contextual nature of revelation and the special psychological conditions that he reportedly experienced during revelation.
In regards with the fourth parameter, the fact that the conflict between science and faith has been largely non-existent in the history of Islamic civilization but science and rational thinking were richly cultivated by a large number of Muslim scholars and scientists under the inspiration of Islam’s own emphasis on observation, intellection and pursuit of knowledge are features that can point to the superior intellectual substance of Islam.
Alright! Although I may be able to write more arguments, I feel what I wrote is sufficient for my purposes. So this must sum it up. Note that some of the above arguments could be debatable especially those that are based on more controversial aspects of Islamic history but they were mainly intended to show how the framework I proposed can be used. I hope you and other users find the effort put into this answer useful and inspiring! And hey this probably made the longest answer on several SE sites on the record! Peace!