Ibn Sina's metaphysics is essentially concerned with ontology, the study of existence, and all the distinctions about it. It occupies a central role in his metaphysical speculations. The reality of everything depends on its existence. And knowledge of an object is, ultimately, knowledge of its ontological status in the universal chain of being that determines all its attributes and qualities.
Everything in the universe, by virtue of the fact that it exists, is subsumed under being. However, God or Pure Being, who is the Origin and Creator of all things, is not the first term in a continuous chain, and therefore, has no "substantial" and "horizontal" continuity with the beings in the world." Rather, God is "earlier" than the universe and is transcendent in relation to it.
Pure Being is God, as understood by the Abrahamic religions. He is God not only as recognised by the Muslim followers of Ibn Sina, but also by Jewish and Christian philosophers, who shared a common conception of the Supreme Being and who, like Ibn Sina, reformulated the teachings of Greek philosophy in monotheistic terms.Ibn Sina's study of the existences that are shared by all things without reducing them to a genre that is common among them cannot be separated from two fundamental distinctions that characterise his entire notion of ontology. These distinctions relate to the essence or quiddity (mahiyah) of things and their existence (wujud) on the one hand and their inevitability, possibility and impossibility on the other. Whenever a person thinks about something, immediately, in his frame of mind, he will be able to distinguish between two different aspects of that something:
Its essence or quiddity, all of which will be covered in the answer to the question, what is something (quid est or ma hiya);
Existence. For example, when someone thinks about a horse. In his mind, he will be able to distinguish between the idea of the horse, or its quidities, which include its state, colour shape, and everything else that makes up its essence and its existence in the external world.
In the mind, quidities are not tied to existence. That is, one can think of or visualise the quidities of an object without paying any attention to whether it exists or not. But, in the external world, the quidity and existence of every object are the same. They are not two components with their own external reality that are combined to form an object, like one would add milk to coffee or water to dough. They exist only in the realm of the mind, in the analyses made by the human intellect. Thus, these two elements become distinct and he realised that every object in the universe has a quiddity to which existence is added.
Having made the basic distinction, Ibn Sina asserted that although the existence of something is added to its essence, it is existence that gives reality to every essence or quidity. Hence, it is the principle (ash!). The quiddity of every thing is essentially nothing more than its ontological limitation abstracted by the mind.