Why can't federal courts just make up law?
Under the doctrine of Erie Railroad Co. v. Tompkins (1938), there is no general federal common law. Although federal courts can create federal common law in the form of case law, such law must be linked one way or another to the interpretation of a particular federal constitutional provision, statute, or regulation (which in turn was enacted as part of the Constitution or after). Federal courts lack the plenary power possessed by state courts to simply make up law, which the latter are able to do in the absence of constitutional or statutory provisions replacing the common law. Only in a few narrow limited areas, like maritime law, has the Constitution expressly authorized the continuation of English common law at the federal level (meaning that in those areas federal courts can continue to make law as they see fit, subject to the limitations of stare decisis).
Much of the study in the madrasah college centred on examining whether certain opinions of law were orthodox. This scholarly process of "determining orthodoxy began with a question which the Muslim layman, called in that capacity mustaftī, presented to a jurisconsult, called mufti, soliciting from him a response, called fatwa, a legal opinion (the religious law of Islam covers civil as well as religious matters). The mufti (professor of legal opinions) took this question, studied it, researched it intensively in the sacred scriptures, in order to find a solution to it. This process of scholarly research was called ijtihād, literally, the exertion of one's efforts to the utmost limit."