Abu Yusuf
He served as the chief judge ("qadi al-qudat") during reign of Harun al-Rashid. His most famous work was "Kitab al-Kharaj", a treatise on taxation and fiscal problems of the state. Biography. Abu Yusuf lived in Kufa and Baghdad, in what is now Iraq, during the 8th century. His genealogy has been traced back to Sa'd b. Habta, a youth in Medina in the time of the Prophet, and his birth date is estimated based on the date of his death to be around 113/729CE. Based on anecdotal stories, Abu Yusuf was raised poor but with  a ferocious appetite for knowledge. His mother disapproved of his academic desires, insisting that he master some trade (the art of tailoring, according to some source) so as to help make ends meet. While it cannot be fully verified, stories suggest that he complied with his mother's wishes, but also kept up his academic studies. His talent and commitment was eventually recognized by Abu Hanifa who became his mentor with Abu Yusuf as his star pupil. He is portrayed as an incredibly studious individual who was unceasing in his pursuit for knowledge and legal understanding. While much of  what is known of his early childhood relies on sometimes contradictory anecdotal evidence, it has been verified that he studied religious law and traditions in Kufa and Medina under a number of scholars including Abu Hanifa, Malik b. Anas, al-Layth b. Sa'd and others. Under the guidance of Abu Hanifa, Abu Yusuf achieved incredible success and helped develop and spread the influence of the Hanafi school of Islamic law. Abu Yusuf lived in Kufa until he was appointed Qadi in Baghdad. It is unclear whether he was appointed by Mahdi, al-Hadi, or Harun al-Rashid. According to one  story, Abu Yusuf was able to provide sound advice pertaining to religious law to a government official who rewarded him generously and recommended him to the caliph, Harun al-Rashid. He continued to provide satisfactory legal opinions to the caliph who drew him into his inner circle and eventually appointed him Qadi. While this version of events is probable, it is not necessarily authentic and cannot be independently verified. What is known is that Abu Yusuf became a close acquaintance of Abbasid caliph, Harun al-Rashid, who eventually granted him the title of Grand Qadi, or Qadi 'l-qudat ; the first time such a title had been conferred upon someone in Islamic history. While at the time it was meant as an honorific title, the Caliph frequently consulted Abu Yusuf on legal matters and financial policy and even bestowed upon him the ability to appoint other Qadis in the empire. This made the position of Grand Qadi analogous to a modern-day chief justice. Abu Yusuf held the position of Grand Qadi until his death in 182/798CE. Literary works. During his lifetime, Abu Yusuf created a number of literary works on a range of subjects including Islamic jurisprudence, international  law, narrations of collected traditions ("ahadith"), and others. The "Kitāb al-Fihrist", a bibliographic compilation of books written in the 10th century by Ibn al-Nadim, mentions numerous titles authored by Abu Yusuf. With one exception, none of these works listed in the "Fihrist" have survived. The exception is his book entitled "Kitāb al-Kharāj", a treatise on taxation and financial issues facing the empire written at the request of the caliph, Harun al-Rashid. The Islamic empire was at the height of its power at the time of his writing and in his treatise, he sought to advise the  caliph on how to appropriately conduct financial policies in accordance with religious law. While the caliph took some suggestions and ignored others, the overall effect was to limit the ruler's discretion over the tax system. A selection of other works credited to him that do not appear in the "Fihrist" have also survived. The "Kitab al-Athar" is a collection of Kufian traditions ("ahadith") which he narrated. "Kitab Ikhtilaf Abi Hanifa wa Ibn Abi Layla" is a comparison of the opinions between the legal authorities, Abu Hanifa and Abu Layla. "Kitab al-Radd ‘Ala Siyar al-Awza’i" is a  reasoned refutation with broad systematic developments, of the opinions regarding the laws of war of the famous Syrian scholar, al-Awza’i. Some excerpts from his various other works that have not survived in their totality were incorporated in texts written by his disciples and were passed on through succeeding generations. For example, excerpts from Abu Yusuf's book, "Kitabal-Hiyal" (Book of Legal Devices) were incorporated in the book, "Kitabal-Makharidj fi 'l-Hiyal" written by his disciple, Muhammad al-Shaybani. Doctrine and Methodology. As a disciple of Abu Hanifa, Abu Yusuf's doctrine largely presupposes that of his mentor . His writings and prominent political positions helped advance the Hanafi school of Islamic law throughout the Islamic empire. While most of his legal opinions ("fatwas") were firmly rooted in the doctrine and methodology espoused by his former teacher, there are some points on which he diverged and revealed his own legal thought. The doctrine of Abu Yusuf was more dependent on traditions ("ahadith") than his master, in part because there were more authoritative prophetic traditions available to him in his time. He also reacted against the somewhat unrestrained reasoning exhibited by Abu Hanifa. However, he was not always consistent; in  a certain number of cases he disregarded sounder and more highly developed doctrine by diverging from the opinions of his former teacher. Based on his surviving works and opinions, certain tendencies in Abu Yusuf's reasoning have been determined, such as his tendency to logically follow the implications of a proposition to an absurd conclusion ("Reductio ad absurdum") and his use of rather caustic language in his attacks on opponents' positions and in defense of his own. Abu Yusuf is also noted for the frequency in which he changed positions on various issues, which has been suggested is a result  of his experience as a judge. Abu Yusuf's greatest legacy is in affirming and advancing the Hanafi legal school as the predominant source of legal thought in the Islamic empire and providing a legal framework for defining and restricting caliphal authority in regard to fiscal policy.