In the philosophies of Mulla Sadra and Tusi, the intellect is the most supreme faculty of the soul, and intellectual knowledge is the highest level of knowledge. Both of them agree that rationality is the most fundamental of all human characteristics, with the difference that the main criterion for Mulla Sadra is theoretical rationality, while for Tusi it is practical rationality. The present paper is devoted to a comparative study of the two concepts of the intellect and knowledge from the viewpoints of these two philosophers.Although Tusi believed in the relationship between the rational soul and the Active Intellect and considered it as the mediator of the soul's moving from potency to actuality, he does not accept its union with the Active Intellect. However, Mulla Sadra stipulates that the human rational soul unites with the actual intellect and maintains that the perception of this problem, like believing in the union of the intellect and the intelligible, is a divine gift. He has faith in the unity of human intellect and is also aware of the nature of knowledge and its end and subject. In fact, his belief in the connection of sciences and their correlation in certain aspects is rooted in the same awareness. Mulla Sadra believes that the reality of knowledge is the same as existence, as knowledge means the appearance of known things and their affixation in souls or minds. Unlike Tusi, he considers perception and knowledge as a kind of a multi-level and graded existence and argues that all of its levels and grades, including sense perception, imaginal perception, and intellectual perception, are immaterial. In his view, the intellect consists of all intelligible things. In contrast, Tusi maintains that in acquired knowledge man's relationship with the external object is through the form developed in the human mind. However, Mulla Sadra is of the view that, like ignorance, knowledge is divided into simple and compound types. Through referring knowledge to being and existence, he has transformed the domain of the common disagreements concerning the quid dative nature of knowledge.