Abu Hanifa al-Numan ibn Thabit worked as a silk merchant in Kufa, in present day Iraq, and taught law as a private scholar rather than as a judge, a distinction his students noted as unusual among the early jurists. Kufa in his lifetime, the first half of the eighth century, was one of the great centers of Islamic legal learning, further from Medina and the dense concentration of hadith recorded there than the schools that developed in Arabia itself, and its jurists developed a method that leaned more heavily on structured reasoning to extend the law to cases the primary texts did not directly address.
Two tools mark the method associated with his name. Qiyas, analogical reasoning, extends an existing ruling to a new case that shares its underlying rationale. Istihsan, usually translated juristic preference or equitable discretion, permits a jurist to depart from a strict analogy when it would produce a result the purposes of the law do not support, substituting a different, better supported ruling.
Critics in other schools regarded istihsan as reasoning too freely from personal judgment; Hanafi jurists defended it as a control on legal formalism, keeping rulings answerable to the law's underlying aims.
Abu Hanifa himself left no systematic legal treatise. The doctrine that bears his name was compiled and extended by his students, chiefly Abu Yusuf, who later served as chief judge, qadi al-qudat, under the Abbasid caliph Harun al-Rashid, and Muhammad al-Shaybani, whose books became the school's core texts. It was through their writing, and the offices they held, that a private teacher's method became a school of law with the institutional weight of the state behind it.
Sources The Canonization of Islamic Law: A Social and Intellectual HistoryAhmed El Shamsy with Principles of Islamic JurisprudenceMohammad Hashim Kamali