History of Islam/The Mutazilites-Asharites debate

1. Basic tenets of the Mutazilite teaching and its early endorsement by Abbasid Caliphs
According to the well know sources on Mutazilism, there are five basic doctrines or principles of Mutazilism. The first is the Divine unity, al-tawhid in Arabic. The second is the Divine justice, al-`adl. The third is the Divine promise and threat, al-wa`d wa-al-wa`id. The fourth is the place between the two places, al-manzilah bayn al-manzilatayn. The fifth is commanding the good and prohibiting the evil, al-amr bi al-ma`ruf wa-al-nahy `an al-munkar.
The first Mutazilite tenet, the Mutaziltes understand that God is One and His oneness makes Him differ from all existent beings created by Him. The Mutazilites deny any similarity between God and His created creatures. Due to their understanding of God's oneness, the Mutazilites interpret metaphorically all the quranic verses indicating any similarity or likeness between God and His creatures such as the Koranic verses stating about God's hands, eyes, sitting on the throne and face. For the Mutazilites, the quranic anthropomorphic verses should be interpreted metaphorically because if those verses of the quran are understood literally, they attribute some similarities between God and His creatures. The literal understanding of the Koranic anthropomorphic verses is very much against the tenet of God's oneness in the eyes of the Mutazilites.
God's transcendentalism is indeed emphasized emphatically by the Mutazilites. They are very strict and serious against the anthropomorphists, the mushabbihah or the mujassimah, among Muslims. Because of their strict adherence to God's transcendentalism, their opponents called them al-mu`atillah (those who denied God's attributes). The Mutazilites replied to their opponents that they themselves do not deny God's attributes but what they indeed deny from God is His attributes are different from His essence. For the Mutazilites, God's essence does not differ from God's attributes. In other words, God's attributes are identical and inseparable from His essence. For this type of understanding regarding the relationship between God's essence and His attributes, the Mutazilites say and uphold that God is powerful by His essence, and in Arabic said, Allah qadir bi-dhatih. Meanwhile, the opponents of the Mutazilites like the Asharites understand and state that God is powerful with His power, Allah qadir bi-qudratih.
In line with their understanding of God's transcendentalism, the Mutazilites deny that God can be seen even by the peoples of Paradise in the hereafter. The Mutazilites argue that if the peoples of Paradise can see God, God must be in any material form since human eyes can only see the material things or forms. Since God is a non-material being, human eyes are impossible to see Him even in the Paradise. The Mutazilites do not differentiate between the natures of human beings in this world and their natures in the hereafter. For the Mutazilites, human eyes in this world are impossible to see God as evident from the Prophet Moses wanted to see God with his own eyes and he could not see God. What is impossible for human eyes to see in this world is also impossible for human eyes to see in the hereafter. For the Mutazilites, human eyes need light, proper distance and location to see the seen object. Since God is unseen being, He cannot be seen by human eyes. It is impossible for human eyes to see God because God is an immaterial being, unseen being.
The Mutazilites deny the Christian concept of Divine trinity. The Trinity in Christianity states the three persons of the Godhead namely the Father, Son, and Holy Spirit. For the Mutazilites, the principle of the Trinity is contrary to God's transcendentalism that stresses nothing similar or identical with God either in His essence or His attributes.
The Mutazilites deny that the Koran is eternal and the speech of God. For them, the Koran was created by God like other created creatures. The Mutazilites deny the uncreatedness of the Koran because for them only God is eternal. Other beings are temporal, transient and finite. God is eternal and infinite. They want to defend the Divine unity when they deny the uncreatedness of the Koran. The Koran is limited and finite because it consists of letters and sentences understood by human minds. The Koran was revealed for human beings. Human beings are limited and finite. Whatever is created by God is limited and finite. Hence the Koran is limited and finite because it was created by God to be the book of guidance to all human beings.
The second tenet of the Mutazililtes is the Divine justice. They understand that God is indeed just and fair to all created creatures. To prove their understanding of God's justice, the Mutazilites advocate the concept or principle of human free will. The concept stresses that man is free to choose and act on what he has chosen. Nothing forces man either to do or not to do if he decides either to do or not to do. Man is free to carry on his intended plan or intention. The Mutazilites totally disagree with the Jabarites who advocate human predestination. The Mutazilite concept of human free will is very identical and similar with the Qadarite concept of human free will. The Qadarites are the opponents of the Jabarites in their understanding of the human power and will and God's power and will. For the Jabarites, man has no will and power to act on his own choice or preference. Man has to rely or depend on God's will and power before he is able to will and act. For the Qadarites, man has his own will and power created by God for himself to will and act or not to will and not to act. Moreover, man is free to will and to act on whatever he wills. If man decides on certain actions, he can act on them. With the power created by God in man's nature, man is capable of willing and acting or doing according to his own wishes. Since man is free from any compulsion, he consequently deserves the rewards or punishments based on his choices and actions. If man does the bad and evil actions, he deserves the punishments from God, and if man does the good and useful actions, he deserves the rewards from God. The Mutazilites deny any injustice attributed to God. God is just and He does not want or He does not will injustice. Injustice takes place because of human beings, not because of God. Indeed, God does not do injustice to any human being.
To defend the Divine justice, the Mutazilites deny the concept or the principle of Divine intercession in the hereafter. For the Mutazilites, the Divine intercession contradicts the Divine justice. The Divine intercession in the hereafter is said to be for the Muslims who committed the grave sins and tney became the grave sinners before they died. For the Mutazilites, God will not intercede or help the grave sinners and God will put them in the Hell because they chose to commit the grave sins prohibited by God. God will not help them. God will judge them according to their own deeds, bad or good deeds. In other words, because of the Divine justice, God will punish the grave sinners in the Hell. The opponents of the Mutazilites like the Asharites uphold the view that God will intercede the grave sinners and take them out from the Hell after they have completed their punishemnts in the Hell. 
In line with the Divine justice, God does the better and best things for human beings. For examples, God created human beings in the best form of human bodily structure in comparison with animals. God granted human beings with minds that are not available among animals. God sent His prophets to guide human beings and sent down His books of revelations to human beings from time to time until the last and final revelation revealed to the last and final prophet named Muhammad in the seventh century in Arabia. He was sent by God to all human beings. 
The Mutazilite concept of God's doing the better and the best things for human beings is known in Arabic as fi`l al-salah wa-al-aslah lil-nas. God showed human beings the two different ways or broadways namely the good and the evil ways. The good way is leading to the good life in this world and then in the hereafter. Meanwhile, the evil way is leading to the evil and miserable life in this world and then in the hereafter. In other words, God showed and proved to human beings the two opposite ways, God's way and Satan's way. Human beings who follow God's way are going to the Paradise in the hereafter and human beings who follow Satan's way are going to the Hell in the hereafter. Human beings have no any reason or word to blame God for their own bad and miserable fates or destinations in this world and then in the hereafter since God did the better and the best things for human beings. God's just and fair actions are evident and apparent to all human beings.
The third Mutazilite tenet is the Divine promise and threat. This one is very much to complement the second Mutazilite tenet. According to the Mutazilites God promises human beings with the rewards if they obey God's commands and God threats human beings with the punishments if they disobey God's commands. God keeps and fulfills all His promises to all human beings. God also will punish all human beings who have disregarded and disobeyed God's commands. There is no possibility of thinking and assuming that God does not keep His promises. The Mutazilites understand that God makes the promises and He will fulfill them. God makes the threats and He will accomplish His threats. For this third tenet of the Mutazilites, the opponents of the Mutazilites like the Asharites claimed that the Mutazilites have imposed on God the duty or obligation to fulfill His promises and to carry out His threats. The Mutazilites reply to their opponents that God Himself imposes on Himself to keep all His promises and to carry out all His threats. Hence human beings cannot claim that God does not keep His promises. Not keeping the promise is bad among human beings and in God's divine law, and God does not do bad action. Fulfilling the promise is good and commanded by God's divine law and God fulfills His promises because it is a good thing to do. God does not do the bad, evil and ugly things. Human beings do all the bad, evil and ugly things if they choose to disobey God's commands. 
The fourth Mutazilite tenet is the place between the two places. It means that the Muslims who have committed the grave sins cannot be placed among the Muslims nor among the disbelievers. The grave sinners have to be placed between the Muslims and the disbelievers. The grave sinners cannot be equated with the disbelievers because they still have faith in their hearts while the disbelievers do not have any faith in their hearts. The grave sinners cannot be equated with the Muslims because they have committed the grave sins prohibited by God. Hence, the Mutaziltes introduce the third place for the Muslims who have committed the grave sins. Their place is between the disbelievers and the Muslims who do not commit the grave sins.
The fifth Mutazilite tenet is commanding the good and prohibiting the evil. This one is for human beings to attain the rewards from God since God promises human beings with the rewards if they do the good actions in this world. Commanding human beings to do the good actions is a good action. Prohibiting human beings from doing the evil actions is a good action. If all Muslims command themselves and others to do the good actions, they are all good Muslims who deserve to attain what God has promised for them. If all Muslims prohibit themselves and others from doing the bad and evil actions, they are all good Muslims who deserve to attain what God has promised for them. The fifth Mutazilite tenet is related to commanding the good and prohibiting the evil among human beings for human peaceful life and prosperity in this world. If all Muslims implement the Mutazilite fifth tenet, there would be no crimes and criminals in all Muslim societies.
The Abbasid caliphs known in Muslim history who have endorsed the five basic tenets of Mutazilism are al-Ma'mun, al-Wathiq and al-Mu`tasim.
'2. Reaction of the scholars and the development and basic tenets of the Asharites'
The Mutazilite-Asharite debate took place between the student and teacher of Mutazilism. The teacher was Abu Ali al-Jubbaii who was the leader of the Mutazilite school of Basra. His student was Abu Hasan al-Ashari. Al-Ashari was not convinced by his teacher regarding the Mutazilite concept of free will and the Mutazilite concept of God's doing the better and the best things for human beings. 
According to the Mutazilite concept of free will, man is free to decide and strive for his future and final destination. If man does the good deeds in his life and dies, he is surely going to attain the good life in the hereafter i.e. he will be in the Paradise. It is obligatory upon God to reward the good man with the good or better life in the Paradise. If man does the evil deeds in his life and dies, he is surely going to be punished in Hell. It is obligatory upon God to punish the evil man with severe and painful punishment in the Hell. In other words, man is free to choose and act on his choice of good or evil deeds and he will be rewarded or punished accordingly depending on his good or evil choices and deeds. God does not make man a believer or a disbeliever. Man himself decides to be a believer or a disbeliever of God's existence and oneness and then to follow God's commands to do the good deeds and to avoid the evil deeds.
To dispute and refute the two Mutazilite concepts above, al-Ashari posed to his teacher the cases of three brothers whose fates or final destinations were totally different. The first brother lived and died as a believer. The second brother lived and died as a non-believer or an infidel. The third brother died when he was still small or minor. 
The first brother was going to the Paradise because he chose to become the faithful or the believer and he did the good deeds. So the case of first brother was in line with the Mutazilite concept of free will and the Mutazilite concept of God's doing the better and the best things for human beings. God surely rewards the believer with the good and better life in the Paradise. In this case, the first brother is in the Paradise because of his good faith and good deeds.
The second brother is going to be in the Hell because he chose to disbelieve and did the evil deeds. He is in Hell because of his choice and free will. He chose and did the evil faith and deeds and the unavoidable consequence of his choice is the Hell in the hereafter. For the second brother, al-Asharite asked his teacher, what was the better or best thing that God did to him? God permitted the second brother to live and die as an infidel and then to be in the Hell. His teacher replied that God punishes the second brother because he chooses the evil faith and deeds in his life in this world. God is just and His justice makes Him punish the wrong doer and reward the good doer.
The third brother is neither in the Paradise nor in the Hell because he did not have enough time to become a believer like his first brother or to become an infidel like his second brother. Again, al-Ashari asked his teacher, what was the better and best thing that God did to the third brother? His teacher replied that God knows the best for him to die when he was still small or a minor because if the third brother were to grow up, he would become an infidel or a non-believer like his second brother. Hence, it is better for the third brother to die when he was still small or a minor.
Al-Ashari said that why did God prolong the life of the second brother even though God knows that he grew up and old and died as an infidel? Being an infidel is not good for the second brother. If the Mutazilite concept of God's doing the better and the best things for human beings was true, as the Mutaziltes claimed, there would be no single infidel living in this world since an infidel is not good to live in this world. In the hereafter, an infidel is going to be in the Hell. God must make all human beings believers because the final rewards for the believers are surely the Paradise in the hereafter, if the Mutazilite concept of God's doing the better and the best things for human beings is an acceptable and reliable concept. In reality, infidels or non-believers are more than believers living in this world. Hence, the Mutazilite concept of God's doing the better and the best things for human beings does not agree with human history in this world.
Although al-Ashari had been the student of al-Jubaii for almost forty years according the very popular story or narrated tradition about their student-teacher relationship, finally al-Ashari debated and disputed the two Mutazilite concepts namely human free will and God's doing the better and the best for human beings. Al-Ashari was doubtful about the two Mutazilte concepts and he soon left his teacher al-Jubaii. Al-Ashari later on known as the founder of the Muslim theological school named Asharism that opposed many Mutazilte concepts and principles of faith or belief.