Every science examines the personal accidents of the subject, which it determines as its subject, and the sciences differ from each other in this aspect. However, sciences offer the principles that they prove themselves through the use of other sciences. Sciences take and use the principles of other sciences that they will need, but they do not have to prove these principles in themselves. On the contrary, they entrust this proof to the sciences from which they took their principles. While examining the sources of fiqh and the method of making judgments from these sources, the method of fiqh uses the principles set forth in other sciences in the problems it encounters. One of the sciences in which the method of fiqh takes principles is kalām. Actually, the science of kalām provides principles to all other sciences with the conception of God and the world it reveals. One of these principles is the indeterminist understanding of the world, in other words, the theory of ʽādah (custom). The theory of custom, produced in the tradition of Ahl al-Sunnah Kalām, is a theory that cancels causality and makes all effects in the world peculiar to Allah. According to this theory, Allah operates the universe according to the laws of physics, which are named custom. However, this theory makes it possible for Allah, if He wills, to do extraordinary acts that are described as humanly extraordinary. Since the Fiqh and its Methodology, which is assisted by the science of kalām, is a science that is expected to judge according to the apparent, it seems to be contrary to his identity to make judgments by using the possibilities of the theory of custom. However, like the situation of a person who becomes a Muslim at the last moment of prayer, sometimes things get so tight that it becomes very difficult to judge according to outward appearance. Well, did the methodists look at the outward appearance and decide that this person is not responsible for the prayer of the aforementioned time, or did they come to a different conclusion by using the possibilities of the theory of custom. This stud will try to determine to what extent the theory of custom, as a theory produced in the tradition of Ahl al-Sunnah Kalām, is used in Fiqh and its Method in terms of the situation of the person who became a Muslim at the last moment of prayer, through the work of Fakhr al-Islām al-Pazdavī (d. 1089) on fiqh method named Kanz al-Wusūl ilā Ma‘rifat al-Usūl and his commentators Siġnākī’s (d. 1314) work named al-Kāfī Sharh al-Pazdavī and Abdulaziz al-Bukharī’s (d.1330) work named Kashf al-Asrār ‘an Usūl Fakhr al-Islām al-Pazdavī by using the understanding-interpretation method. This study is important in terms of understanding the details of the relationship between kalām-fiqh and its methodology. Actually, in classical works, a general portrait is drawn about the aspect of the method of fiqh to get help from the science of kalām, and it is mentioned that the arguments used in the science of methodology depend on the existence of Allah and the proof of the Prophet’s accuracy. However, it is not mentioned in detail that other principles set forth in the science of kalām can also be used. In this context, our study is important in terms of dealing with the relation of fiqh and its methodology with kalām, as a theory of custom, in particular. Thus, it will be possible to determine to what extent the general expressions in classical works are applied in detail, specific to the direction we are examining. In addition, our study is of great importance in terms of determining whether the use of this theory in the context of the above contradictory situation causes a problem in terms of the identity of judging according to appearance. In the direction of the findings, it has been determined that the Methodists benefited from the theory of custom as a requirement of the "khalafiyyah" system, which they established, but they did not reflect this tasthe reality. We can call this behavior purposeful sense because the khalef comes to the fore in the absence of the original together with the possibility of the original. It can be understood from the fact that the methodologists, who obtained this possibility from the theory of custom, did not believe that the situation was like this and that they concluded that the decree of khalef was immediately valid. In addition, it seems that the fact that they make use of the theory of custom in a purposeful sense and do not reflect this on reality does not seem to harm the identity of judging according to the outward appearance of fiqh and its methodology.