Islamic concepts

ʿIllah — effective or operative cause, reason

Term used in legal theory

the common ground that you apply case number 1 to case number 2

Thus, according to the majority position, a ruling of the Shari‘ah is always present so long as its ‘illah is present and even if its hikmah is absent. Moreover, a ruling of the Shari‘ah is absent if its ‘illah is absent even if its hikmah is present.

There are four essential elements of qiyas called its arkan, or pillars:

  1. The asl, original or principal case;93
  2. the far‘, a novel case;94
  3. the ‘illah, which is a feature (wasf), which must be found to be shared by both the asl and far’; and lastly,
  4. hukm al-asl [hereinafter hukm], the ruling of the principal case that is extended from the asl to the far‘ if the ‘illah is common to both.95

Jurists of all legal schools, except the Hanafis,96 agree upon these four elements. The Hanafis consider the ‘illah to be the essential pillar of qiyas and the remaining three elements to be conditions of qiyas, not its constituents.97

Islamic legal system

http://islamicsystem.blogspot.com/2011/03/difference-between-al-illah-and-al_18.html