The Muʿtazila, ahl al-sunnah or shiʿah ʿAli
In today’s parlance, individuals who stick to the past are called “traditionalists” (sunnatī). The word Sunnī which means sunnatī refers to the one who follows the tradition (sunnah) and does not subscribe to thinking (fikr) and intellection (taʿaqqul). However, since the sunnah was associated with the Prophet’s Sunnah, the terms acquired a sense of sanctity. [...]
The Sunnīs as they are called today are not the opposite of the Shīʿah. We think that the Sunnī is he who believes in Abū Bakr as the Prophet’s (S) immediate successor and the Shīʿah is he who does not believe in it. It is not so. In the beginning, the word Sunnī was not in contradistinction to Shīʿah; it was in contradistinction to Muʿtazilah. When it was said, “He is Sunnī ,” it meant that he was not a Muʿtazilite. But today we call both Muʿtazilites and Ashʿarites “Sunnīs.”
At that time which was the time of the Imams, the Shīʿah were with the Muʿtazilites in this respect and not with the Ashʿarites. The word Sunnī was juxtaposed against Muʿtazilah, but since the Shīʿah shared some things with the Muʿtazilites [the] word Shīʿah was later juxtaposed against Sunnī after the Muʿtazilites were persecuted and, among the advocates of intellection, only the Shīʿah remained. Thereafter, a way of thinking emerged which perhaps associated the word Sunnī to the question of caliphate although it has nothing to do with its root. Yet, the word gradually acquired this meaning.
In the millennium congress on Shaykh al-Ṭūsī held in Mashhad, Ᾱqā Ḥajj Mīrzā Khalīl Kamare’ī made an interesting observation on the word Sunnī, a good word applied by the Ahl al-Sunnah to themselves. He said:
“They defrauded us in the dark. They came and chose this beautiful name for themselves, saying, “We are Sunnis which means that we follow the Sunnah of the Messenger of Allah (S) and that others do not follow the Sunnah of the Messenger of Allah (S).” This is while the contention is not about the Sunnah of the Messenger of Allah (S) in that one would say, : The Sunnah of the Messenger of Allah (S) is authoritative” while other would say it is not authoritative. Even the Muʿtazilites would not say, “We do not accept the Sunnah of the Messenger of Allah (S).” The contention is on the quality of inference and treating the Qur’an, the Sunnah and the intellect. If we set aside the intellect, we cannot also understand the Qur’an and the Sunnah, and this argument is correct.”
The Shī ʿah world was immune from this [Ibn Ḥanbal’s] literalism and the reason for this, as I have explained, was that this movement emerged during the time of the Imams (A) who were very close to the Muʿtazilites in this respect such that in history the Shīʿah and the Muʿtazilites are mentioned together and are called ʿadliyyah (justice-advocates). Many people even called the Shīʿah Muʿtazilites and called the Muʿtazilites Shīʿah.
Murtada Mutahhari, Training & Education in Islam (translated by Mansoor Limba), (London: ICAS Press, 2011), 275-278.