By the ninth century, the major Sunni schools of law had each settled on some combination of Quran, hadith, consensus and qiyas, analogical reasoning, as their sources, extending a scriptural ruling by likeness to cases scripture never directly addressed. Dawud ibn Khalaf al-Zahiri, born around 815 and active chiefly in Baghdad, broke with that consensus on principle. He argued that qiyas amounted to substituting human guesswork for divine law, and that juristic preference and personal opinion, however learned the jurist, had no proper place in deriving legal rulings at all. What remained, in his system, was the zahir, the plain apparent sense of the Quran and authentic hadith, applied as directly and as literally as the text allowed, with consensus accepted only where it was genuinely unanimous and explicit rather than inferred.
The approach won him a following, and for a time the Zahiri school operated as a recognized fifth current alongside the four schools that survive today, though it never achieved their lasting institutional backing from any ruling dynasty. Its adherents were sometimes accused by rival jurists of a rigid, almost mechanical literalism that ignored the underlying purposes scholars believed the law was meant to serve, a charge Zahiri jurists rejected as itself an act of the unauthorized human reasoning they had set out to exclude.