Precisely because tawhid is fundamental, the attempt to state it exactly produced the sharpest theological quarrels in early Islam. The central problem was the status of God's attributes named in the Qur'an, his hearing, seeing, speech, his "hand" and his sitting on the throne. If these are real and distinct from his essence, does that not compromise his oneness? If they are denied, is the Qur'an not being contradicted?
The Mu'tazila pressed strict unity: God's attributes are identical with his essence, and, their most contested claim, the Qur'an is created, since an eternal, uncreated speech alongside God would be a second eternal thing. Under the caliph al-Ma'mun this view was briefly imposed by an inquisition, the mihna (from 833), in which scholars were compelled to profess the created Qur'an. The traditionists resisted, and the Ash'ari and Maturidi schools that became mainstream Sunni theology affirmed that God has real attributes, "not he and not other than he," and that his speech, the Qur'an, is uncreated, accepting the difficult formulations bila kayf, "without asking how." A separate current, the wahdat al-wujud ("unity of being") associated with Ibn Arabi, pushed tawhid toward a mystical monism and drew lasting criticism from jurists who read it as blurring the line between Creator and creation.