Those who distinguished between fundamentals and subsidiary matters did not provide a criterion separating the two categories. Sometimes they say, “This is certain, while that is probable.” Yet many questions of practical rulings are certain, while many questions of fundamentals are probable in some people’s estimation. Whether something is certain or probable is a relative matter. At other times they say, “Fundamentals concern knowledge and reported truths, while subsidiary matters concern practice.” Yet denying many practical obligations constitutes unbelief, such as denying the obligation of prayer, zakat, fasting, and Hajj. Sometimes they say, “These are matters known through reason, while those are known through revelation.” But if they are matters known through reason, it does not follow that someone who errs concerning them must be declared an unbeliever, for unbelief is a judgment of revealed law and depends upon that law. This has been discussed at length elsewhere.

When a person reflects on people’s disagreements, he finds that each group possesses knowledge that the other lacks, just as happens in questions of practical rulings.

An example is the preceding discussion of the five fundamentals: divine unity, justice, the intermediate position between the two positions, questions of religious designations and rulings, and the execution of the threatened punishment. These are the principles on which the Muʿtazilah base their allegiance to those who agree with them and their disavowal of those who disagree. We have already explained that they intended to affirm the Lord’s oneness, His justice, wisdom, mercy, and truthfulness, and obedience to His commands. Yet they erred concerning each of these matters, as explained above.

The same applies to those who opposed them among the Jahmiyyah and those who followed their approach, such as Abū al-Hasan al-Ashʿarī and his followers: they opposed them concerning the five fundamentals, and they possessed knowledge