In this article, the problem of universals is discussed in terms of the relationship between knowledge and being, on the basis of the works of Ibn Sīnā and his commentators. Problems pertinent to the discussion of universals stem mainly from Ibn Sīnā’s establishing the concept of essence (māhiyya) as something to be contemplated (i‘tibār), and then employing essence as a thing (shay’). In Ibn Sīnā’s philosophy, all the determined conditions of the essence in itself are mixed with accidents, or specific instantiations (arad). The status of absoluteness, where the essence is unmixed with accidents, becomes manifest only in one’s mental consideration; therefore, there is no external and mental space where an essence is completely abstracted from external and mental accidents. However, an impression is created in which the essence is made to look like part of an external “individual” when the process of the occurrence of an essence, along with that of its abstraction from individuals, is depicted. The arguments of Ibn Sīnā led the commentators to the reconsideration of two fundamental problems regarding universals in a way compatible with the theory of essence. The first problem pertains to the existence of a universal in the mind, and the second problem to its existence in the external world. The first problem relates to the nature of knowledge, and the second problem concerns itself with the reality of a thing that exists in the external world. In connection with those problems, Fakhr al-Dīn al-Rāzī’s assessments in particular –which worked out the implications of Ibn Sīnā’s philosophy and early theological speculations– determined the sub-aspects of the problem of universals by giving a new direction to the discussions about the arguments of Ibn Sīnā, offering a different perspective of the problems regarding the nature of what is known and the nature of an existing thing. The main question that this article seeks to answer is what the relation between knowledge and the known according to the works of Ibn Sīnā and his commentators? To answer that question, the article primarily draws attention to a set of problems contained in the discussion of universals and the contexts in which Ibn Sīnā addresses them. It then considers various commentaries on the arguments of Ibn Sīnā, such as those offered by Fakhr al-Dīn al-Rāzī, Na~īr al-Dīn al-Tūsī, Qutb al-Dīn al-Rāzī, and Sayyid Sharīf al-Jurjānī.