Hey! This is a super interesting question. A few years ago I wrote about this, so I'm going to draw from that and then expand on it to answer a few of the other questions you have. If you don’t want all of this additional information, the next comment down will summarize it and an answer to your question (but if you came to /r/AskHistorians, I would hope you want more rather than less!)

---

You first have to understand the background of early Islamic law and the debates between two of the major camps, the mutakallimūn and the muḥadithūn.

If we wanted to apply modern English equivalents, you could possibly call them the "Rationalists" and the "Traditionalists", respectively (just note that these terms can't be seen as a 1-to-1 replacement for the original). Wael Hallaq, one of the foremost scholars on Islamic law, highlights the rationalist as one who “does not rely, or tends to not rely, upon ḥadīth”, instead opting to rely upon raʾy, or independent reasoning. As such, he pits the rationalists (mutakallimūn) and their belief in the values of human reason squarely against the traditionalists (muḥadithūn), who “held that law must rest squarely on Prophetic ḥadīth”.

Islamic Law is interesting in that there are relatively few laws or commandments contained within the Qur'an itself. For example, while the Qur'an may give information about prayer and fasting, it doesn't cover all scenarios. Moving on to more "legal" matters, such as the punishment for crimes, the Qur'an only states the punishment for around 5 crimes. These ḥudūd crimes (coming from the word for "border" because they're strictly defined) are adultery, false accusation of adultery, drinking alcohol, highway robbery, and some forms of theft.

When attempting to model a society on Islamic morals, this obviously presents a problem. How is one to apply the broad-strokes of the Qur'an to individual cases? What is the punishment for cheating a business partner with whom you had a contract? Or accidentally burning down your neighbors home? This is where the conflict between the mutakallimūn and the muḥadithūn come in. Because Islamic law isn't codified, the individual qādīs, or judges, had pretty extensive lee-way when it came to deciding cases. Depending on if they agreed with the rationalist or traditionalist approach, the same case could have a very different outcome. This lack of homogeneity in decisions actually leads to some super interesting cases of "shopping around" for a more favorable judgement, but that is a topic for another time.

When confronted with a case that hadn't appeared before, a traditionalist judge would consult the sunna and ḥadīth. What did Muhammad say about this subject, even if it wasn't in the revelation of the Qur'an? How did those who were closest to Muhammad act? As they viewed Muhammad as the highest example of man, they believed it logical to attempt to emulate him as much as possible. Meanwhile, a rationalist would be more willing to use his individual reason, raʾy, and a process known as qiyās. Qiyās is similar to the process a modern lawyer in a common-law country does, looking back at past judgements and ḥadīths to create a new injunction for a new scenario. For example, smoking isn't outright prohibited in the Qur'an or Hadith. However, smoking is proven to be bad for you. The Qur'an forbids Muslims from harming themselves. Ergo, smoking should be forbidden. While qiyās can be applied by rationalists and traditionalists, it was far more commonly used by rationalists, adapting the law to their individual scenarios even when there may be a previously existing case that was similar.

In the first few centuries of Islam, raʾy, which had been commonly employed among qāḍīs to determine how to best make religious rulings, began to be seen in an increasingly negative light. Initially, the use of independent reason had proliferated in Early Islam, and upwards of two-thirds of cases hinged upon the qāḍīs use of raʾy. Indeed, when presented with a case for which he could not find precedent in the sunnah, the second Caliph ʿUmar delegated to the qāḍī presiding over the case full authority to employ his own raʾy in coming to a solution. The use of raʾy by not only a caliph, but a companion of the Prophet, shows how widely accepted its application in the Early-Islamic world was.

However, as the sunnah and the ḥadīth became compiled into reliable and constant collections, such widespread use of personal interpretation soon came under attack from the more traditionalist muḥadithūn Muslim thinkers. The discretionary reason that was so widely employed under ʿUmar came to be seen as arbitrary, and as evidence of man attempting to put his own thoughts above the precedent that God had provided for humanity. The rationalists who frequently employed raʾy began to be referred to, sometimes derogatorily, as ahl al-raʾy (People of Opinion) in contrast to the traditionalists self-designation of ahl al-sunnah (People of the Sunna).

The gap between the traditionalist muḥadithūn and the rationalist mutakallimūn continued to grow over the next century and a half. Early Muslim judges did not possess the sort of bureaucratic power many may associate with judges today. Their rulings did not constitute authoritative precedent and there was no formal institution that granted them the ability to preside over cases. Rather, their authority came from their epistemic-moral authority within the community; the reputation that they had built for themselves as pious, learned men capable of applying sharīʿa properly. Over the course of the conflict between the two schools of the ʿulamaʾ, the proto-Sunni muḥadithūn began to actively degrade the moral legitimacy of their opponents, the mutakallimūn. They issued statements prohibiting prayer with or being married by so-called “theological deviants” and began to purge the legacy of transmitters unsympathetic to their ideals from the isnād. These actions can be seen as concerted attacks to reduce the epistemic-moral authority of the mutakallimūn thinkers. The intra- ʿulamaʾ conflict between these two schools thus provides the cornerstone, but not the entire foundation, upon which the miḥna was based. The miḥna itself was a sort of inquisition led by a Caliph sympathetic to rationalist ideals, in which he attempted to force scholars to believe in the creation of the Qur'an. Here, we are starting to get to your question.

The conflict between rationalism and traditionalism was part of a larger debate - one regarding the interpretation of Islam as seen by the caliphal court and as imagined by the urban ʿulamaʾ - that struck at the heart of the delineation between political and religious authority. Ira Lapidus, along with John Abdallah Nawas and numerous other scholars, posits that the Caliph al-Maʾmūn (r. 786 – 833 C.E) deliberately adopted the rationalism of the muʿtizila school of thought due to its emphasis on the creation of the Qurʾan (khulq al-Qurʾan). Adopting khulq al-Qurʾan would not have changed the position of the Qurʾan as a religious document, something accepted by both the rationalist and the traditionalist schools. However, if the Qurʾan was seen as created and not eternal, it would have been placed under the earthly purview of the Caliph. If the ʿulamaʾ capitulated and accepted this doctrine, it the caliph would have successfully asserted his religious authority over them. In essence, Caliph al-Maʾmūn’s insistence on khulq al-Qurʾan would have constituted a power grab to consolidate religious and political power in a singular figure.

To facilitate extending his authority, the Caliph al-Maʾmūn launched the miḥna, commonly referred to in English as an ‘inquisition’, in the final four months of his rule in 833 C.E.. Within the letters that he wrote to declare this miḥna, which lasted with varying degrees of intensity through the reign of two of his successors until 848 C.E., al-Maʾmūn makes clear references to his high position and the weight that it gives his actions, asking that God direct him “to just actions in the exercise of political authority over his people”. This phrasing leaves little doubt as to the political leaning of the miḥna trials. The Caliph made clear note throughout his letter that he was more than a mere political sovereign, but rather possessed not only the capability but the duty to regulate the field of religious debate. Thus, the miḥna marks the entrance of the political into a field previously restricted to intra-religious debate between the muḥadithūn and the mutakallimūn.

When the Caliph al-Maʾmūn attempted to force a particular byproduct of mutakallimūn reason onto religion, khulq al-Qurʾan, he was in essence introducing the political sovereign as a “competing institution” to the strict traditionalism of the muḥadithūn branch of the ʿulamaʾ. Wael Hallaq credits the Qurʾan as providing a “cosmology entirely grounded in moral natural laws”. It provides a framework inherently stronger than the morally relativistic laws of Europe, instituted at the whims of the political sovereign currently in power. This assumes however, that the Qurʾan exists beyond the material world and cannot be influenced by man. If khulq al-Qurʾan was successfully adopted, the protected status of the Qurʾan would be threatened; it would no longer be “embedded . . . in a cosmology that comprised a metaphysic” but rather it would become a physical, corruptible document.

---

Continued in the next post