القياس
al-Qiyās
Topic
Reasoning by analogy from the Quran and the Sunnah, as a source of Islamic law.
What it means
Al-Qiyās is logical deduction applied to the sources of Islamic law. When no text of the Quran or of the Sunnah, nor consensus (ijmāʿ), settles a new case, the jurist compares it with a case that a text does settle, finds the shared reason (ʿillah), and extends the ruling to the new case. Most Sunni jurists count it as the fourth source of law, after the Quran, the Sunnah and consensus, and the schools differ on how widely they apply it. It is the main means by which the law meets circumstances the early community did not face.
Often misunderstood as
That jurists invent law from opinion, when qiyās works from a stated reason found in the sources.
Term and Arabic as listed in Ismail al Faruqi, Toward Islamic English (International Institute of Islamic Thought, 1995), p. 52, listed as AL QIYĀS. The definition is restated here.