How do scholars reach rulings on matters that the Qur'an and the Sunnah do not address directly? Classical Islamic legal methodology, called usul al-fiqh, recognises the Qur'an and the Sunnah as the two primary textual sources. The majority of scholars add two further tools: consensus, called ijma', and analogical reasoning, called qiyas.

Consensus refers to the agreement of qualified scholars on a ruling. It is most authoritatively understood as the consensus of the Companions, or of the qualified scholars of a given generation. Once genuine consensus is established on a matter, it is treated as binding. But whether a true, complete consensus actually exists on a specific point is itself something scholars can disagree about.

Analogical reasoning is the process of extending a known ruling to a new case that the texts do not address directly, on the basis of a shared underlying reason. A commonly cited example is the Qur'an's prohibition of wine, which scholars extended to other intoxicating substances, because both intoxicate the mind.

Ijtihad is the disciplined effort of a qualified scholar to derive a ruling from these sources, where the answer is not already settled by clear text or established consensus. It requires deep knowledge of Arabic, of the Qur'an and Sunnah, of the established rulings of earlier scholars, and of the methodology of the school being applied.

This is different from taqlid: following the ruling of a qualified authority. Following a qualified authority has been the practical norm for the great majority of Muslims, including most scholars outside their own area of specialism.

Not every scholar or school has weighted these sources identically. The Zahiri school, historically associated with Dawud al-Zahiri and later Ibn Hazm, is known for limiting or rejecting analogical reasoning, preferring to rely more strictly on the apparent wording of the Qur'an and Sunnah. It is mentioned for completeness; it is not among the four schools most Muslims follow today.

This is a simplified overview of a technical field that classical scholarship treats in far greater depth. It is meant to give a general sense of the method, not to equip anyone to perform ijtihad themselves.