Al-Shafii's Risala sets out a hierarchy of exactly four sources of Islamic law, the Quran, the Sunna, consensus and analogy, and argues that a jurist has no fifth instrument available once these four have been properly worked through in order. Al-Risala fi Usul al-Fiqh itself presents the ordering as following from the nature of revelation and reason rather than as an arbitrary convenience, the Quran and Sunna supplying the primary textual material, consensus confirming what the community has settled beyond dispute, and analogy extending settled rulings to new cases that share the same effective cause. The Canonization of Islamic Law: A Social and Intellectual History records that the significance of the list lay less in its specific four items, several of which earlier jurists had already used in some form, than in al-Shafii's insistence that the list is closed and that no source outside it, including juristic preference, may function as an independent instrument of law. That insistence on closure is what later tradition points to when it credits al-Shafii with founding usul al-fiqh as a stated discipline, since a method is only a method, capable of being checked and taught, once its own limits have been declared rather than left open to whatever a given jurist finds persuasive in a particular case.