In the history of human thought, several issues and problems have been propounded by thinkers. Some of them have played a fundamental role in solving the philosophical or social problems of people and determining their path of life. However, some of them have been merely abstract ones with no clear relation to human aspects and modes. The principality of quiddity and existence and the gradation of being are among the issues in Islamic philosophy that have attracted the attention of Islamic philosophers and thinkers. Although not explicitly mentioned in Peripatetic Philosophy, they have been of great significance in Ishraqi wisdom (Illuminationist Philosophy) and the Transcendent Philosophy. Based on the mentality dominating the principiality of existence and gradation of being, one might ask what epistemological effects result from this issue.The writer of this paper, employing a library method of research, has tried to explain some of the philosophical and theological effects and consequences of these issues. From among all of them, we can refer to a correct explanation of oneness (tawhid), whether in essence, attributes, or acts, the causality of God, the commensurability and conformity between the cause and effect, and individuation, the justification of the existence of copula, the quality of attaining knowledge, and the relation between the world of nature and the world of intellects (the emanation of pluralities), and the clarification of predication in propositions.