أن لا يكون المعدى حكماً لا يعقل معناه
The ruling being extended must not be one whose operative meaning is inaccessible to reasoning.
Some legal prescriptions specify a quantity or form as an act of worship, without exposing a transferable effective cause. A superficial resemblance cannot be used to extend such a ruling.
Crucial distinction: Suggesting a possible wisdom behind a rule does not establish its legally recognized ʿillah. An act of worship may have many benefits, yet those benefits do not authorize a jurist to change its required number or generate a new rule by analogy.
Analogy applies where a legally transferable cause can be identified.