This article based on the analysis of his third, little-known work of the Arabic historian and geographer of the 9th century al-Ya’qubi “Mushakalat an-nas li zamanihim” (The Adaptation of men to their time). It reveals new aspects of Al-Ya’qubi’s oeuvre. The treatise relatively recent work composed by al-Ya’qubi that has reached after two of his well-known works “Kitab al-buldan” (Book of countries). and "Kitab at-ta’rih (“Book of History”). In the article, this treatise is considered from a sociological perspective. Al-Ya’qubi, based on the thesis that all people in Muslim society followed their caliphs, imitated their conducts and behaviors, is trying to identify the causes of the degradation of Muslim society during the reign of Umayyads and early Abbasids. He considers that the main reason for the moral and spiritual decay of Muslim Society are the caliphs themselves. People imitated the customs of the caliphs followed their spiritual and moral orientation. Thus, the quality of Muslim society, according to al-Ya’qubi, directly depends on its leader. As far as the caliph, in its moral, ethical and spiritual qualities, corresponded to the image of the “Amir al-Mu’minin” – “Commander of the Faithful,” so did the members of society correspond to the image of the faithful Muslims. The treatise reveals new aspects of al-Ya’qubi’s worldview, his vision of a just, righteous Muslim society. Through the analysis of this treatise, a new side of al-Ya’kubi’s oeuvre is revealed - sociology. At first glance, an attempt by al-Ya’kubi to delve into the reasons for the development of Muslim society may seem primitive. However, al-Ya’qubi’s work, considering the development of Muslim society through the prism of “adaptation of men to their time”, could be believed the first example of a medieval Arabic work on sociology.It follows from the article that the thesis and examples of “the adaptation of men to their time” should be considered more deeply. The very division of the caliphs (rulers) of Islam into the categories of “caliphs” and “kings,” as can be seen from the author's introductory remarks to the treatise, says a lot. From the context of the treatise, it turns out that the author lists the first four righteous caliphs as “caliphs,” distinguishing among them Osman ibn Affan, who, according to his description, is deprived of the asceticism and modesty of the two previous caliphs Abu Bakr and Omar ibn al-Khattab. By “kings” of Islam, the author means the caliphs of the Umayyad and Abbasid dynasties. Thus, the main plotline of the treatise is based on the opposition of a righteous Muslim society, led by the “caliph” to its “perverted” form (according to al-Ya'qubi) – the Muslim empire, the “caliphate”, under the authoritarian rule of the “kings” in the person of the caliphs – Umayyads and Abbasids.