Example: someone might contend that usury applies to maize by analogy with rice, and that it applies to rice by analogy with wheat. Such a chain of analogies is invalid; rather, one should say that usury applies to maize by analogy with wheat, so that the branch is measured against an original whose ruling is established by a textual proof.
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The ruling in the original case must rest on a known effective cause (ʿillah), so that the original and the branch can be linked through it. If the ruling in the original is purely devotional and no causal rationale is known, analogy may not be drawn from it.
Example: a person claims that eating ostrich meat nullifies ablution by analogy with camel meat, arguing that the two animals are similar. This analogy is unsound, because the ruling concerning camel meat has no known effective cause; according to the predominant view it is purely devotional. -
The effective cause must contain a meaning that is suitable and relevant to the ruling, a relevance recognized by the overall principles of the law—such as intoxication in the case of wine. If the purported cause is merely an invariable attribute with no juridical relevance, it is not valid—blackness and whiteness, for instance.
Illustration: In the hadith of Ibn ʿAbbās—may Allah be pleased with them—concerning Barīrah, she was given a choice regarding her husband when she was emancipated, and the narrator said, “Her husband was a black slave.” (1) The word “black” is a mere incidental attribute with no relevance to the ruling. Therefore a slave-girl who is freed while married to a slave retains the option even if he is white, yet she does not have that option if she is freed while married to a freeman, even if he is black. -
The effective cause must be present in the branch to the same degree that it is present in the original;