VIII.—QIAS.
Qi′a′s (lit. "to compare") is the fourth foundation of Islám, and expresses the analogical reasoning of the learned with regard to the teaching of the Qurán, Hadís, and Ijmaʾ.
There are four conditions of Qiás: (1) That the precept or practice upon which it is founded must be of common (ʾamm) and not of special (kháss) application; (2) The cause (illat) of the injunction must be known and understood; (3) The decision must be based upon either the Qurán, the Hadís, or the Ijmaʾ; (4) The decision arrived at must not be contrary to anything declared elsewhere in the Qurán and Hadís.
Qíás is of two kinds, Qíás-i-Jalí or evident, and Qíás-i-Khafí or hidden.
An example of Qíás-i-Jalí is as follows:—Wine is forbidden in the Qurán under the word Khamar, which literally means anything intoxicating; it is, therefore, evident that opium and all intoxicating drugs are also forbidden. Qíás-i-Khafí is seen in the following example:—In the Hadís it is enjoined that one goat in forty must be given to God. To some poor persons the money may be more acceptable; therefore, the value of the goat may be given instead of the goat.