The adherents of the Mutazili school, known as Muʿtazilites, are best known for denying the status of the Qur'an as uncreated and co-eternal with God, asserting that if the Quran is the word of God, he logically "must have preceded his own speech".

The philosophical speculation of the Muʿtazilites centred on the concepts of divine justice and divine unity.The school worked to resolve the theological "problem of evil": how to reconcile the justice of an all-powerful God with the reality of evil in the world.It believed that since God is just and wise, he cannot command what is contrary to reason or act with disregard for the welfare of his creatures.

Muʿtazilites believed that good and evil were not determined by revealed scripture or interpretation of scripture, but they were rational categories that could be "established through unaided reason; because knowledge was derived from reason, reason was the "final arbiter" in distinguishing right from wrong.

The Muʿtazili school of Kalam considered the injunctions of God to be accessible to rational thought and inquiry and that reason, not "sacred precedent", is the effective means to determine what is just and religiously obligatory.

The movement emerged during the Umayyad caliphate and reached its height during the Abbasid caliphate. After the 10th century, the movement declined. It is viewed as heretical by some scholars in modern mainstream Islamic theology for its tendency to deny the Qur'an being eternal.

In contemporary jihadism, the epithet or supposed allegations of being a Muʿtazilite have been used between rival groups as a means of denouncing their credibility.

The name Muʿtazili is derived from the reflexive stem VIII (iftaʿala) of the triconsonantal root ع-ز-ل "separate, segregate", as in اعتزل iʿtazala "to separate (oneself); to withdraw from".

The name is derived from the founder's "withdrawal" from the study circle of Hasan of Basra over a theological disagreement: Wāṣil ibn ʿAṭā' asked about the legal state of a sinner: is a person who has committed a serious sin a believer or an unbeliever? Hasan answered the person remains a Muslim. Wasil dissented, suggesting that a sinner was neither a believer nor an unbeliever and withdrew from the study circle. Others followed to form a new circle, including ʿAmr ibn ʿUbayd. Hasan's remark, "Wāṣil has withdrawn from us", is said to be the origin of the movement's name.

The group later referred to themselves as Ahl al-Tawḥīd wa l-ʿAdl (اهل التوحيد و العدل, "people of monotheism and justice",[citation needed] and the name muʿtazili was first used by its opponents.

The verb i'tizal is also used to designate a neutral party in a dispute (as in "withdrawing" from a dispute between two factions). According to the Encyclopædia Britannica, "The name [Mutazilah] first appears in early Islāmic history in the dispute over ʿAlī's leadership of the Muslim community after the murder of the third caliph, ʿUthmān (656). Those who would neither condemn nor sanction ʿAlī or his opponents but took a middle position were termed the Muʿtazilah." Nallino (1916) argued that the theological Mu'tazilism of Wasil and his successors was merely a continuation of this initial political Mu'tazilism.

Origin
Mu'tazili theology originated in the 8th century in Basra (Iraq) when Wāṣil ibn ʿAṭā' (d. 131 AH/748 AD) left the teaching lessons of Hasan of Basra after a theological dispute regarding the issue of al-Manzilah bayna al-Manzilatayn (a position between two positions).

Though Mu'tazilis later relied on logic and different aspects of early Islamic philosophy, ancient Greek philosophy, and Indian philosophy, the basics of Islam were their starting point and ultimate reference. The accusations leveled against them by rival schools of theology that they gave absolute authority to extra-Islamic paradigms reflect more the fierce polemics between various schools of theology than any objective reality. For instance, Mu'tazilis adopted unanimously the doctrine of creation ex nihilo, contrary to certain Muslim philosophers who, with the exception of al-Kindi, believed in the eternity of the world in some form or another. It was usually Muslim philosophers, not the Muslim theologians generally speaking, who took Greek, Indian, and Hellenistic philosophy as a starting point and master conceptual framework for analyzing and investigating reality.

This school of thought emerged as a reaction to political tyranny; it brought answers to political questions, or questions raised by current political circumstances. The philosophical and metaphysical elements, and influence of the Greek philosophy were added afterward during the Abbasid Caliphate. The founders of the Abbasid dynasty strategically supported this school to bring political revolution against Umayyad Caliphate. Once their authority was established, they also turned against this school of thought.

Historical development
Like all other schools, Mu'tazilism developed over an extensive period of time. Abu al-Hudhayl al-'Allaf , who came a couple of generations after Wasil Ibn 'Ata' and 'Amr ibn 'Ubayd, is considered the theologian who systematized and formalized Mu'tazilism in Basra. Another branch of the school found a home in Baghdad under the direction of Bishr ibn al-Mu'tamir ; the instigators thought it was the Caliph's own scheme:under Ma`mun the Great , "Mu`tazilism became the established faith. The Mu`tazilites maintained, like the Qadarites of the later Omayyad period, man's free will, also that justice and reason must form the foundation of the action God takes toward men, both of which doctrines were repudiated by the later orthodox school of the Ash`arites."

The persecution campaign, nonetheless, cost them their theology and generally, the sympathy of the Muslim masses. As the number of Muslims increased throughout the Islamic empire, and in reaction to the excesses of this newly imposed rationalism, theologians began to lose ground. The problem was exacerbated by the Mihna, the inquisition launched under the Abbasid Caliph al-Ma'mun. Ahmad ibn Hanbal, a Muslim jurist and founder of the Hanbali school of Islamic jurisprudence was a victim of Ma'mun's Mihna. Due to his rejection of Ma'mun's demand to accept and propagate the Mu'tazila creed, ibn Hanbal was imprisoned and tortured by the Abbasid rulers. Under Caliph Mutawakkil (847-861), "who sought to reestablish the traditional Moslem faith" (intentionally wanted to restore his legitimacy due to backlash towards Ahmad Ibn Hanbal's persecution under previous Caliphs), Mu`tazilite doctrines were repudiated; their professors persecuted; Shi`ites, Jews, and Christians were also persecuted."

In response to the attacks, Mu'tazili theologians refined and made their idea system more coherent and systematic Jackson (2002) argued against the "fiction" of that there was a strict traditionalist vs. rationalist dichotomy between the theological mainstream and mu`tazilah, asserting that much rather that traditionalism and rationalism, in the Islamic context, should be regarded as "different traditions of reason."

In Basra, this task was accomplished by the father and son team, Abu 'Ali al-Jubba'i and Abu Hashim al-Jubba'i . The two differed on several issues and it was Abu Hashim who was to have the greatest influence on later scholars in Basra, including the prominent Abd al-Jabbar ibn Ahmed who became the most celebrated proponent of Mu'tazilism in the late tenth and early eleventh century. Mu'tazilism did not disappear from the Islamic intellectual life after the demise of 'Abd al-Jabbar, but it declined steadily and significantly. By the end of the 15th century, Mu'tazilism had largely faded into obscurity within Sunni circles and was rarely maintained openly as theological position, though Mu'tazilite positions remained an integral aspect of Imami and Zaidi Shi'ite theological doctrines up until the present day and Mu'tazilism itself has even seen a gradual revival in modern times in spite of deeply ingrained prejudices within the contemporary Muslim world.