Ebû Cafer es-Simnânî (öl. 444/1052), bugün İran sınırları içerisinde yer alan Simnân’da doğmuş, Bâkıllânî başta olmak üzere birçok isimden ilim tahsil etmiştir. Musul’da kadılık görevindeyken vefat eden Simnânî, fıkıhta Hanefî, itikadî açıdan ise Eş‘arî mezhebini benimsemesiyle dikkat çekmektedir. Bâkıllânî’nin öğrencisi olması, Bâcî’nin hocaları arasında yer alması ve en önemlisi el-Beyân ‘an usûli’l-îmân adını taşıyan eserinin günümüze ulaşması, Simnânî’nin kelamî kişiliği açısından geniş yelpazede bilgi sunmaktadır. Simnânî’nin eseri, yaşadığı zaman dilimi ve coğrafyanın din anlayışıyla ilgili bilgiler vermekle birlikte yazarın Ehl-i Sünnet telakkisini ortaya koyması açısından dikkate değerdir. Eserin Mu‘tezile mezhebine reddiye niteliğinde yazılması, dönemin iklimini yansıtması açısından önemlidir. Bu çerçevede Ebû Cafer es-Simnânî’nin Mu‘tezile eleştirisi ya da Mu‘tezile algısı ve Ehl-i Sünnet telakkisi makalenin konusu olarak belirlenmiştir. Tarih ilmi yöntemini temel kabul eden çalışma, şahıs ve eser üzerinde derinleşerek, Simnânî ve eserinin İslam düşünce geleneğindeki yerini tespit etmeyi amaçlamaktadır. Makalenin sonuç olarak “mezhepler arası ilişki” üst başlığında literatüre katkısının olduğunu ve özellikle “Eş‘arîlik-Mu‘tezile mücadelesi” çerçevesinde önemli bilgiler sunduğunu ifade etmek gerekir.Abū Djafar al-Simnānī (d. 444/1052) was born in Simnān, which is located within the borders of Iran today and received knowledge from many names, especially Abū Bakr al-Bāqillānī. Simnānī, who died while he was a judge in Mosul, draws attention with his adoption of Hanafī in fiqh and Ash‘arī in kalam. It is understood that this union is not unique to him and that his family has such a vein. It should be noted that Simnānī is an exceptional identity as he combines Hanafism and Ash‘arism sects in a single body within the framework of inter- sectarian interaction. The fact that his works on fiqh have not reached the present day and even the names of his works are not known obscures his fiqh personality. Some evaluations can be made on this subject only based on his duty as a judge and his relationship with the Dāmaghānī family. Compared to Simnānī’s Hanafī identity, Ash‘arī identity can be revealed more clearly. The fact that he was a student of al-Bāqillānī, that he was among Abū al-Walīd al-Bājī’s teachers, and most importantly, that his work named al-Bayān ‘an usūl al-īmān has survived to the present day provides a wide range of information about Simnānī's theological personality. It is known that he has a work on “destiny”, a work on “imāmat” related to Shiism, and a work of kalam other than al-Bayān although they have not survived. Since he was al-Bājī’s teacher, Simnānī’s works and views were transferred to the West and especially to Andalusia. For this reason, it should be stated that Simnānī played a key role in the transfer of Ash‘arism to the West and, understandably, Ibn Hazm put Simnānī in the center in his criticisms of Ash‘arism. With the result of that Simnānī, who lived in the hijri 5th century, is an important and rare scholar in the History of Islamic Sects with his work named al-Bayān ‘an usūl al-īmān wa al-kashf ‘an tamwīhāt ahl al-tughyān. In addition, Simnānī is an important link of the “Bāqillānī tradition” when the different veins in Ash‘arism are taken into account. In the fourth stage of the (1) Ash‘arī (2) Bāhilī and Tāī (3) Bāqillānī (4) Simnānī (5) Bājī line, together with Abū Dhaar al-Harawī, he has ensured the continuation and strengthening of Ash‘arism tradition. Simnānī’s work is remarkable in revealing the author’s view of the Ahl al-Sunnah and giving information about the period he lived in and the approach of geography. In his book, the author evaluates the scope of Ahl al-Sunnah and his approach to other sects. Simnānī states that he wrote his work for the Mu‘tazila sect at the beginning of his work. Accordingly, it should be accepted that al- Bayān is important in terms of showing the opposition of Ash‘arism to the Mu‘tazila. It should also be noted that the Mu‘tazila portrait drawn by Simnānī in his work, like that of many other authors, contains some errors compared to his sources. The fact that the work is written as a rejection of the Mu‘tazila sect does not mean that no evaluation has been made about other sects. In addition to these, the author has given information about the naming of the sects. Another point that shows the importance of al-Bayān is that it includes some concepts that Simnānī used for the first time, which have a negative content for Mu‘tazila and some Mu‘tazilī scholars. The study, which accepts the methods of historical science as a basis, aims to determine the place of Simnānī and his work in the tradition of Islamic thought by deepening on the person and the work. In his al-Bayān ‘an usūl al-īmān, Simnānī reveals his evaluations of the Mu‘tazila sect and his perception of other sects. It should be stated that the study will contribute to the literature on the relationship between sects and provide important information, especially within the framework of the “Ash‘arism- Mu‘tazila struggle.” In addition, the article provides clues about the existence of names that are outside the generalization regarding Hanafī-Māturīdī and Shāfi‘ī- Ash‘arī identities. As a result, the study reveals the perception of an Ash‘arī who lived in the hijri 5th century about other sects, especially the Mu‘tazila.