Usul al-fiqh has been defined as principles or methodology used by the jurists (mujtahid) to deduce the practical Shari`ah ruling from their sources.
Thus the purpose of usul al-fiqh is to regulate the prosess of adjudication (ijtihad) and to guide the jurist (mujtahid) in his effort at deducing the rules from the sources. This would explain the issue of how do Muslim judges and jurists discover and apply the law.