This study deals with the four-source order, which is commonly accepted in the Sunni legal theory, within the limits of a historical investigation of Islamic legal theory. This article aims to determine in which phases this recognition has occurred and with what sort of changes. The majority of extant uṣūl works that have been produced in different traditions and some other sources in different disciplines have been defined as the research universe of this study. Among them, those which could represent these works have been cited in this article. Since it was designed as a historical investigation and evaluation study, some dimensions of the subject that may be of interest in terms of legal philosophy and theory are not included in the analysis. These topics some of which have already been studied in certain works are suggested for further research. In this article, two main questions are addressed: First, the question of which actors have historically contributed to the acceptance of the four-source order in the form of kitāb, sunna, ijmā' and qiyās. On this point, contrary to what is claimed in some contemporary scholarship, it has been revealed that the thesis that al-Shāfi'ī was the responsible authority for this four-source order is not accurate. It is seen that the scholars living in the second to fourth centuries A.H. mentioned these four sources together with different sources such as reason, definite deduction, sense, language, recurrent reports and famous narrations. Among these, al-Jaṣṣāṣ's explanation as to whether qiyās can be counted as a source together with kitāb, sunna, and ijmā', which is the main subject of discussion in the next periods, has significantly illuminated the rational ground for qiyās to take place in this four-source order. Beginning with the fifth century, the tendency to restrict the notion of dalīl to definitive sources, had a consequence in the direction of excluding qiyās from the apparatus of source order; hence it was generally accepted that the sources were limited to kitāb, sunna, and ijmā' during that time. However, in the same century, it seems that jurist-theoreticians made the claims that qiyās should be recognized together with these three sources. At this point, it seems that the Hanafi theoreticians, especially al-Pazdawī and al-Saraḥsī, took an important step of expanding their acceptance of the main sources by claiming qiyās as the fourth source after mentioning the three main sources as kitāb, sunna, and ijmā'. Al-Sam'ānī, who could be regarded as a contemporary of them and who had been İntihal Taraması/Plagiarism Detection: Bu makale intihal taramasından geçirildi/This paper was checked for plagiarism Geliş/