Author-jurists sought out fatwas that reflected the social conditions of their time and place, often opting for later legal opinions which were at variance with the doctrine of early authorities. Research by Wael Hallaq and Baber Johansen has shown that fatwa compilations could, and sometimes did, have a significant impact on the development of Islamic law. During the early centuries of Islam, the roles of mufti, author-jurist and judge were not mutually exclusive. A jurist could lead a teaching circle, conduct a fatwa session, and adjudicate court cases in a single day, devoting his night hours to writing a legal treatise. Those who were able to act in all four capacities were regarded as the most accomplished jurists. From the standpoint of morality and religious obligation, the term "fatwa" has been contrasted with "taqwa" (piety, fear of God), particularly in Sufi literature. Fatwas may allow a choice between lenient and strict interpretation of "sharia" on a certain matter, or they may employ legalistic stratagems ("hiyal") to circumvent a stricter interpretation, while such strategies may not be acceptable from the standpoint of "taqwa". The basic prerequisite for issuing fatwas under the classical legal theory was religious knowledge and piety. According to the "adab al-mufti" manuals, a mufti must be an adult, Muslim, trusted and reliable, of good character and sound mind, an alert and rigorous thinker, trained as a jurist, and not a sinner. On a practical level, the stature of muftis derived from their reputation for scholarly expertise and upright character. According to legal theory, it was up to each mufti to decide when he was ready to practice. In practice, an aspiring jurist would normally study for several years with one or several recognized scholars, following a curriculum that included Arabic grammar, hadith, law and other religious sciences. The teacher would decide when the student was ready to issue fatwas by giving him a certificate ("ijaza"). During the first centuries of Islam, it was assumed that a mufti was a "mujtahid", i.e., a jurist who is capable of deriving legal rulings directly from the scriptural sources through independent reasoning ("ijtihad"), evaluating the reliability of hadith and applying or even developing the appropriate legal methodologies. Starting from around 1200 CE, legal theorists began to accept that muftis of their time may not possess the knowledge and legal skill to perform this activity.