It is known that Fārābī, in his political program, assumes philosophically some Islamic sciences like kalām and fiqh. Focusing here on the case of the fiqh and in the limits of his K. al-Milla, we try to establish correspondences between his theory of legislation and references to historically attested sciences. Our purpose is to show that he was able to articulate fiqh to the political science through an undeclared use of ʿilm uṣūl al-fiqh (science of principles of juridical science). He empties this science of its own material and preserves its form in order to fill it with philosophical material. This contributes to clear up his conception in this book of the shift from the voluntary universal to the particular. This shift is governed by a set of formal rules. These guidelines, which are rather deliberative, take place as a subject matter of the political science, alongside with universals, its classical subject matter. We think that these formal rules are borrowed from this typical Islamic science whose subject matter is the study of principles and rules governing the inference, from their sources, of particular legal status, the subject matter of fiqh. If, from the strict philosophical point of view, political science provides the sources or foundations from which are inferred the original and primary legislation initiated by the founder of the religion (al-milla), as well as the derived and secondary one elaborated by his successors, the methodological part of this Islamic science would be the equivalent to the general prescriptions necessary for the application of universals so that concrete cases can be determined. For this purpose, Fārābī borrows willingly from the famous Shāfiʿī’s Risāla, the prototype of the treatises in uṣūl al-fiqh.