QIYᾹS ("analogy") is a method of reasoning that entails the extension of a precedent to an essentially similar situation. One of the four principal sources of law among Sunnī Muslims, qiyās was the last to gain explicit recognition, and then only after a fierce controversy that has left its mark on the history of Islam. The expansion of the territorial domains of Islam after the great conquests raised an increasing variety of issues not covered by the Qurʾān or the sunnah (tradition of the prophet Muḥammad). Islamic jurists, therefore, felt the need to have recourse to reason, logic, and opinion. Their freedom was, however, limited. In a society committed to the authority of the revelation, the use of personal opinion (raʾy) in religious and legal matters evoked opposition. In theory, the Qurʾān contained a complete revelation and, supplemented...