In legal methodology (usul al-fiqh), the problem of the defining istiasan and the legitimacy of its definition is among the critical discussion topics. To overcome the rigorism of law, istihsan was used by the founder scholars of Hanafi school of law and Malik b. Anas (d. 179/795)-however this use received various objections. Although the Maliki scholars also used istihsan strong criticisms of istihsan have been directed on the Hanafis. After the severe criticism of Muhammad b. Idris al-Shafii (d. 204/820), the Hanafi scholars carried out various intellectual activities to define istihsan and to prove that it is legitimate. This process-which started especially with Abu'l-Hasan al-Karkhi (d. 340/952) and Abu Bakr al-Razi al-Jassas (d. 370/981), revealed a rich accumulation with the contribution of the legal theorists in other school of laws. It is known that al-Karkhi's definition of istihsan has become widespread among the legal theorists, and the legitimacy of istihsan in the works of legal theory was based on his definition. AlKarkhi's considerations on the legaliticamy of using istihsan can be traced in his student al-Jassas, in the following centuries, Abu Zayd al-Dabusi (d. 430/1039), Abu l-'Usr al-Bazdawi (d. 482/1089) and Shams al-Aimma al-Sarakhsi (d. 483 /1090)'s works. Al-Jassas' explanations on the legitimacy of istihsan begin with the criticism of Imam al-Shafii's against istihsan. He provides the proof of legitimacy for istihsan in divided two sub-headings. Al-Jassas, who deals with istihsan in two parts as word and meaning, argues that there is no disagreement about the use of istihsan and that all mujtahids accept istihsan, and even Imam al-Shafi'i, who opposes to the use of istihsan, employs istihsan in this sense. The type of istihsan on which there is a deabte is the istihsan used in relation to its meaning. Al-Jassas states that this use is a qiyas operation; he justifies his point of view by referring to the works of Muhammad b. Hasan al-Shaybani (d. 189/805). Indeed, when we look at how Imam Muhammad explained the istihsan's examples in his works, it is seen that there are two conflicting juristic analogy (qiyas) made in many examples. Moreover, Imam Muhammad openly considered some types of istihsan part of qiyas by stating that some examples "can be included in this type of istihsan."