Usul al-fiqh is the discipline of how you get from a text to a ruling, and its founding statement is al-Shafii's Risala, written around 820. It sets out four sources — the Quran, the sunna of the Prophet, the consensus of the scholars, and reasoning by analogy — and, crucially, makes the method explicit and arguable. Once the method is on the table, disagreement stops being deviance and becomes jurisprudence: the schools differ over how much weight to give a solitary report, whether local custom or public interest can generate rulings, and how far analogy extends, and they reach different answers by declared routes. The Hanafis lean on juristic preference, the Malikis on the practice of Medina and public interest, the Hanbalis on the texts most narrowly. All four Sunni schools recognise the others as legitimate, which is one of the most consequential settlements in Islamic history and the reason the tradition could absorb Andalusia, Java and the Sahel without fracturing. The modern crisis is not disagreement but authority: mass literacy, the internet and the collapse of madrasa monopolies mean anyone can now issue an opinion, and the discipline that made disagreement orderly is the thing most eroded.