Illah

Illah (Arabic: عِلَّة) is a central concept in Islamic jurisprudence (usul al-fiqh), referring to the underlying rational basis or cause that grounds a legal ruling (hukm). In this theory, the illa is the feature from which a ruling is derived and which can be shared by other cases; when two cases share the same illa, the ruling may be extended to the second case through qiyas (analogy) or confirmed by explicit textual evidence (nass). The illa is distinct from the hukm itself and from the nass; the nass may state the ruling directly, while the illa explains why the ruling applies.

Identification of the illa relies on textual sources and reasoned analysis. Some scholars emphasize clear, explicit illa established in the Qur’an or Sunnah; others allow a rationally discovered illa that remains consistent with the spirit of sharī‘ah. An illa must be a genuine, observable feature that logically necessitates the ruling, not a mere preference or conjecture; misidentifying the illa can lead to flawed conclusions.

A classic example is the prohibition on intoxicants, linked to the illah of intoxication. Since intoxication undermines reason and social order, other substances that cause intoxication are considered prohibited through qiyas, even if not named in scripture. The illa concept thus plays a crucial role in applying rulings to new situations, subject to textual and doctrinal constraints.

Islamic legal schools differ in how strictly they require consensus on the illa, how they treat ambiguous illah, and how they balance illa with textual evidence. Illah remains a methodological tool for connecting established rulings to novel cases, while maqasid al-shariah focuses on broader ethical objectives.