Usul al-Fiqh (Principles of Jurisprudence)

Usul al-Fiqh (plural of asl) is Islamic legal theory — the science of the principles and methods used to derive practical legal rules (fiqh) from the Islamic sources. Imam al-Shafi'i is considered the founder of this discipline through his groundbreaking work "al-Risalah."

The four main sources in usul al-fiqh are: the Quran (Allah's speech), the Sunnah (the Prophet's words, actions, and tacit approvals), Ijma' (scholarly consensus), and Qiyas (analogical reasoning). Additionally, different schools recognize supplementary principles such as istihsan (juristic preference), maslahah mursalah (public interest), and 'urf (custom).

Regarding prayer legislation, usul al-fiqh is crucial for understanding why the schools of law can reach different conclusions. For example: is a particular prayer act obligatory or merely recommended? The answer depends on how one interprets the relevant Quranic text or hadith — and precisely this interpretive method is the domain of usul al-fiqh. Major works in the discipline include al-Ghazali's "al-Mustasfa" and al-Amidi's "al-Ihkam."