The article considers the ethical views of Aristotle and Thomas Aquinas. It focuses on Saint Thomas’s adaptation of Aristotle’s ethical theory. The most important issue in this regard is the structure man. By proposing the essential structure of man, Aristotle stresses his spiritual and physical nature and explains both change and identity in an individual. However, Aquinas changed Aristotle’s theory of substance fundamentally by introducing the concept of existence as the first act of being. The notions of freedom, will, synderesis and conscience are crucial in perceiving man as a subject of moral actions. By introducing the concepts of synderesis and conscience Aquinas changes the representation of man in the context of his actions. By emphasising the will as the power responsible for intellectual desire, he makes it easier to explain the proper improvement of man in choosing the means to reach his final end. After analysing the views of Aquinas and Aristotle one can conclude that the essential view on human structure does not lead to the issue of the person. The existential view of the structure of man makes it possible to explain the immortality of the human soul, and creates a space for relations between God and man. It seems therefore that the different starting points of Aristotle and Aquinas with regard to the structure of man led them to different conclusions in the field of moral philosophy.