The study of existence is the principal pivot of Ibn-Sina's philosophical system. Ibn-Sina"s opinion about existence is based upon the difference hetween "quiddity" (mahiyyat) and "existence" (wujud). This principle is so important that he builds up much of his discussions on theology and ontology upon it. Following al-Farnbi, Ibn-Sina considers "existence" a metaphysical element, distinguished from "quiddity". From his point of view, "quiddity" is exactly the presence of contingent within the knowledge of God. Unless God grants his existence, his essence will never come into existence. Ibn-Sina was well aware of the religious concept of creation. Having been inspired by the religious texts, he establishes a principle in his philosophy that has been followed by the Islamic philosophy thereafter. To know God as the only existent, within whose realm no non-existence is allowed, means the very acceptance of God as the pure existence, and that nothing may be found beyond Him. The logical requirement for such a statement is to prove the existence of God by means of a new reasoning which Ibn-Sina names the veracious reasoning (Borhan Siddigin). It is inconceivable to claim that such a reasoning be presented by philosophers like Plato and Aristotle who do not consider God the very existence, who do not present the relationship between God and universe within the pivot of existence, and who do not believe in creation. This article studies the question of existence from Ibn-Sina's point of view in order to clarify the reason why the metaphysical difference between "quiddity" and "existence" which was neglected by Aristotle, was the main concern of Ibn-Sina; and the reason why this problem, that the Islamic philosophy has taken grand steps in this way and towards its consequences and correctness, was the basis of Ihn-Sina's ontology.