This essay begins with analyzing and criticizing the definition of "Analogy" and then the author presents his definition.He fully examines the four pillars of Analogy, i.e. principle, derivative, major legal status, and the reason, and explains their conditions. The most of the essay discusses ways of exploring the reason and criterion for a legal status. The author analyzes some terms as "Quranic Text, the required meaning indicated by the text, implication, concept, consensus, exclusion, appropriateness, unity of method, refinement of the basis of the ruling".He finally explains in which of the cases one can ignore the principle and refer to the derivatives instead.