Two terms are used to refer to law in Islam: shariah and fiqh. Shariah refers to God's divine law as contained in the Quran and the sayings and doings of Muhammad (hadith). Fiqh refers to the scholarly efforts of jurists (fuqaha) to elaborate the details of shariah through investigation and debate. Muslims understand shariah to be an unchanging revelation, while fiqh, as a human endeavor, is open to debate, reinterpretation, and change.
Scholars and jurists developed the law by combining knowledge of the Quran, hadith, and analogical reasoning with local practice. Beginning in the mid-eighth century, the major Sunni schools of legal thought (madhhabs)—Hanafi, Maliki, Shafii, and Hanbali—and the Twelver Shii Jafari madhhab emerged. Other minor and short-lived schools also developed.
Sunnis and Shiis differed in their understanding of who held the power to interpret shariah. For Sunnis, the scholars had this right, as delegated by the actual ruler. Shiis initially believed that only an imam (in this case, a descendant of Muhammad) could interpret shariah because the imam, like Muhammad, was believed to be infallible. When the line of appropriate descendants ended, this tradition was reinterpreted to grant judicial authority to the fuqaha as the imam's representatives. In addition to the Quran and hadith of Muhammad, Shiis also use the rulings of the imams. Ijma, or consensus, is admitted only if it includes the infallible imam's opinion.
There are two types of fiqh literature: that dealing with usul al-fiqh (roots) and that dealing with furu al-fiqh (branches). Usul al-fiqh explores the four sources of the law—the Quran, hadith, consensus (ijma), and analogical reasoning (qiyas)—to provide structures for interpreting revelation. The Quran and hadith are considered to be equal in authority, although the Quran, as God's word, is superior in its nature and origins. Other issues include the principles of abrogation (naskh); the application, ramifications, and limitations of analogical argument; and the value and limits of consensus. This whole set of interpretative structures is brought together in the idea of ijtihad, or independent reasoning, which both recognizes and encourages a variety of interpretations on all but the fundamental structures of the law. Only those with sufficient educational background in the sources of the law are qualified to practice ijtihad.
Education in fiqh was a critical part of Islamic education from the tenth century forward. It provided training in systematic thought and controlled argument, serving the needs of the merchant classes and governing bureaucracies. In the modern period, exclusive training in the traditional Islamic sciences has become less relevant as legal education has been reconstituted along European lines and the jurisdiction of religious courts has been restricted or eliminated.
Furu literature, both legal manuals and collections of cases, discusses rules for rituals (ibadat) and social relations (muamalat). Ritual topics include purity, prayers, alms (zakah), pilgrimage, fasting, and jihad. Social relations topics include marriage, divorce, inheritance, buying, selling, lending, hiring, gifts, testamentary bequests, agency, deposit, crimes, torts, penalties, compensations for injury, judicial practice and procedure, rules relating to slaves, land ownership, land holding, contractual partnerships, slaughter of animals for food, and oaths and their effects. There are five categories of actions in furu literature: mandatory, recommended, permitted, abhorred, and prohibited.