In Sunni Islamic jurisprudence, the qiyas (Arabic قياس) is the process of analogical reasoning in which the teachings of the Hadith are compared and contrasted with those of the Quran, i.e., in order to make an analogy with a known injunction (nass) to a new injunction. As a result of this method, the ruling of the Sunnah and the Qur'an may be used as a means to solve or provide a response to a new problem that may arise. This, however, is only the case providing that the set precedent or paradigm and the new problem that has come about will share operative causes (illah). The illah is the specific set of circumstances that trigger a certain law into action. Both Sunni Islam and Shi'a Islam share Qur'anic interpretation, the Sunnah, and Ijma' (consensus) as sources of Islamic law, although the two sects differ significantly with regards to the manner in which they use these sources. The sects also differ on the fourth source. Sunni Islam uses qiyas as the fourth source, whereas Shi'a Islam uses 'aql (intellect). Other methods of deducing the law, such as mafhm al-nass (the clear implication of the text), tamthil (similarity or likeness), istihsan (juristic preference), or istislah (consideration of public interest), either explicitly rely on qiyas or use methods of analysis that are similar in their understanding of qiyaas.
1 Example of qiyas
2 Shi'a view of qiyas
Example of qiyas
For example, qiyas is applied to the injunction against drinking wine to create an injunction against cocaine use.
Identification of a clear, known thing or action that might bear a resemblance to the modern situation, such as the wine drinking.
Identification of the ruling on the known thing. Wine drinking is haraam, prohibited.
Identification of the reason behind the known ruling ('illah). For example, wine drinking is haraam because it intoxicates. Intoxication is bad because it removes Muslims from mindfulness of God. This reason behind the reason is termed hikmah.
The reason behind the known ruling is applied to the unknown thing. For instance cocaine use intoxicates the user, removing the user from mindfulness of God. It is therefore prohibited.
During the Islamic Golden Age, there was a logical debate among Islamic logicians, philosophers and theologians over whether the term qiyas refers to analogical reasoning, inductive reasoning or categorical syllogism. Some Islamic scholars argued that qiyas refers to inductive reasoning, which Ibn Hazm (994-1064) disagreed with, arguing that qiyas does not refer to inductive reasoning, but refers to categorical syllogism in a real sense and analogical reasoning in a metaphorical sense. On the other hand, al-Ghazali (1058-1111) (and in modern times, Abu Muhammad Asem al-Maqdisi) argued that qiyas refers to analogical reasoning in a real sense and categorical syllogism in a metaphorical sense. Other Islamic scholars at the time, however, argued that the term qiyas refers to both analogical reasoning and categorical syllogism in a real sense.
Liberal movements within Islam often extend qiyas by the disputed practice of istihsan in order to redefine Islamic law away from conservative and traditional forms.
Shi'a view of qiyas
The Shi'a view the use of qiyas (analogy) as being an innovation which can easily lead the user to erroneous conclusions regarding matters of Fiqh. In Usul al-Kafi, in the chapter on knowledge, one finds many traditions cited from the Shi'a Imams that forbid the use of qiyas. For example:
"H 103, Ch. 11, h 9
Ali ibn Ibrahim has narrated from Muhammad ibn ‘Isa from Yunus from Dawad ibn Farqad from one he narrated from ibn Shubruma (a judge in al-Kufa during the rule of al-Mansur) who has said the following.
"I never heard any thing like a statement I heard from Imam abu ‘Abdallah (a.s.) and it is almost as he said, ‘Pierced my heart.’" The Imam (a.s.) said, "My father narrated from my great-great-great-great grandfather, the holy Prophet (s.a.) who said, ‘Those who act on the basis of analogy will face their destruction and lead others to their destruction. Those who give fatwas without the knowledge of the abrogating and the abrogated, the clear text and that which requires interpretation, they will face destruction and lead others to their destruction."
Conclusion:Qiyas is considered Secondary source of Islamic law.Exercise of Qiyas is allowed where, there in no mentioned in the Holy Quran and Sunnah of Prophet without Qiyas it would be difficult to solve problems of modern society.Qiyas is actually extension of law from the original text.Qiyas help us in discovering law but not to established law.