qiyas, in Islamic law, analogical reasoning as applied to the deduction of juridical principles from the Qurʾān and the Sunnah (the normative practice of the community). With the Qurʾān, the Sunnah, and ijmāʿ (scholarly consensus), it constitutes the four sources of Islamic jurisprudence (uṣūl al-fiqh).
The need for qiyas developed soon after the death of Muhammad, when the expanding Islamic state came in contact with societies and situations beyond the scope of the Qurʾān and the Sunnah. In some cases ijmāʿ legitimized a solution or resolved a problem. Very often, however, qiyas was used to deduce new beliefs and practices on the basis of analogy with past practices and beliefs.
Muslim scholars consider qiyas a specific variant of the general concept of ijtihād, which is original interpretation and thought. It is also related to raʾy, personal thought and opinion, a forerunner of qiyas criticized by traditional authorities as too arbitrary.