The term Mu'tazilah derives from the Arabic al-mu'tazilah, which means the one who separated. It was applied to the school established in Iraq by Wasil b. 'Ata, a student of the distinguished scholar Hasn al-Basri.
At the time of the rise of the 'Abbasids in 750 the Mu'tazilites began to become prominent in the Islamic world. In the 9th century the 'Abbasid caliph, al-Ma'mun, raised Mu'tazilah doctrine to the status of the state creed. Openly supported by the caliphate, the Mu'tazilites became increasingly intolerant and began to persecute their opponents. On one occasion the eminent Sunni scholar and founder of one of the four orthodox jurisprudential schools, Ahmad b. Hanbal (d.855), was subjected to flogging and imprisonment for his refusal to subscribe to the Mu'tazilite doctrine that the Qur'an was created in time.
In fact, the Mu'tazilite school of theology emerged out of the question raised by the Kharijites whether works are integral to faith or independent of faith. On the question of the relationship between faith and works, the Mu'tazilites adopted the position that someone who commits a grave sin without repenting occupies a middle state between being a Muslim and not being a Muslim.
A second doctrine concerned the nature of God. God is pure Essence and, therefore, without eternal attributes such as hands. Passages in the Qur'an that ascribe human or physical properties to God are to be regarded as metaphorical rather than literal. The Mu'tazilites argued that the Qur'an was created and not eternal. The basis of this doctrine was the claim that the eternal coexistence of the Qur'an beside Allah gave the impression of another god beside Allah.
In this theology human acts are free and, therefore, people are entirely responsible for their decisions and actions. Divine predestination is incompatible with God's justice and human responsibility. God, however, must of necessity act justly; it follows from this that the promises of reward that God has made in the Qur'an to righteous people and the punishments he had issued to evildoers must be carried out by him on the day of judgment.
Mu'tazilites are generally seen as responsible for the incorporation of Greek philosophical thought into Islamic theology. This is particularly apparent in their belief that knowledge of God can be acquired through reason as well as revelation.
The Mu'tazilites' power gradually began to wane. They lost the support of the caliphs and by the 10th century the Sunni majority opposition to Mu'tazilah found a spokesman in Abu al-Hasan al-Ash'ari, who himself had previously been a Mu'tazilite. Al-Ash'ari's new school of theology and the school of Abu Mansur al-Maturidi provided the new basis of orthodox Islamic theology, leading to the complete disappearance of the Mu'tazile movement.