Although myth is defined in various forms, but it seems not to have a comprehensive definition. Many of these definitions and categories include indicators such as narrative, symbolic, ideology, holy, transhistorical, sacred, paradisiac, imagination, system, gods, angels and so on. This study has attempted to provide a new definition for myth, epic and tragedy. In this redefinition, myth is defined as some either "unknown or known in different ways concepts"; the unknown concepts that encompass human's mind; always and everywhere accompany him/her; and his/her endless striving is to know and recognize them. Since these concepts are human-like, universal and eternal, like for myths, we can call those unknown concepts of the human mind as myth. This unity solves the problem of various definitions of myth. Epic is "the challenge of man for knowing and teaching those concepts" and tragedy is "the ability or disability of human in knowing and teaching those concepts". Concepts represent their manifestation in phenomena, here referred to as symbols. Acceptance, capacity, deepness and broadness of concepts in the process of myth, epic and tragedy are effective. Didactic literature has a special position to teach and apply these concepts in Iranian culture, which is sometimes derived from the teachings of Islam and sometimes from ancient Iranian culture. Saadi's Golestan, as a valuable example of didactic literature, is an appropriate scope for cognitive analysis (the theory proposed in this article). Saadi's super concept is human, where it challenges the main concept of relaxation, but main concept of greed confronts to it and it has a formidable presence throughout the book, and impedes achievement (and victory). It inhibits him from knowing himself and other concepts (tranquility, fear, death and the like).