Around the second century AH, when oral jurisprudence was about to change to school jurisprudence, the differences of jurisprudents on some issues, both in primary and secondary principles, became apparent, and perhaps the same jurisprudential differences played a major role in establishment of jurisprudential schools in the early Islam. Among the differences, analogy is the most important issue in philosophy and logic, especially in Sunni and Imamiyya jurisprudence that became one of the principles, so that it was counted one of the ijtihadi sources2 in Sunni jurisprudence, one of the ijtihadi proofs3 in Imamiyya jurisprudence, and one of the arguments and argumentation tools, especially in logic and philosophy. This article, in addition to providing a comprehensive definition of analogy, seeks to examine its weakness and strength, and according to logic, philosophy, and the critique of al-Ghazzali’ s views in al-Mustaṣ fa, give a logical answer based on ijtihad to the question “ Why does not Imamiyya act according to analogy? ” . Also, it proves that Imamiya does not act according to analogy, due to the conjectural essence of commonly accepted jurisprudential analogy and the necessity of acquiring certainty in religious rules through the Qur’ an and the conduct of the infallibles (as).