What is the definition of a madhhab, and what is the reason for the اختلاف (disagreement) among madhhabs and the prohibition by some of what others do not prohibit, even though the Sunnah of the Messenger of Allah, peace and blessings be upon him, is one?

In terminology, a madhhab (school of thought) is what a mujtahid (independent jurist) specializes in concerning subsidiary ijtihadi (derived through independent reasoning) legal rulings, which are based on probabilistic evidence. The differences among the A'immah (Imams) who are mujtahidin are a mercy from Allah. This is because Allah Almighty did not establish definitive evidence for all legal rulings; rather, He made them probabilistic, intending to provide latitude for those accountable. Among the reasons for this divergence are:

1. The hadith not reaching some mujtahidin.

2. The Imams' disagreement on the authenticity of a hadith after it has reached them, such as differing on the reliability or weakness of narrators, or differing on the conditions of authenticity.

3. Their disagreement on understanding the hadith or its linguistic ضبط (precise meaning), or whether it is abrogated or definitive, or whether its indication is general or specific, absolute or restricted, or if its intended meaning is literal or metaphorical.

4. Some of them stipulating conditions for khabar al-wahid (a solitary narration) that others do not stipulate.