the name "usuli" derives from the term "uṣūl al-fiqh" (principles of jurisprudence). in usuli thought, there are four valid sources of law: the quran, hadith, ijma' and 'aql. ijma' refers to a unanimous consensus. aql, in shia jurisprudence, is applied to four practical principles which are applied when other religious proofs are not applicable: "bara'at" (immunity), "ihtiyat" (recommended precautions), "takhyir" (selection), and "istishab" (the presumption of continuity in the previous state).