Ontology is the study of Being .
Being is the most general concept. Everything that the word 'is' or its cognates can be applied to may be said to fall under this most general of all concepts. It is that concept that 'something has persistence in manifestation' in some form for some time'.
Being is called the most general concept because it is an abstraction that applies to everything, absolutely everything that ever was, is now or will be in the future. Because it is a characteristic of everything, then it is said to tell us nothing about anything. Thus the concept of Being is at the same time the most empty and the fullest concept. It is full because it touches everything. It seems empty because it says nothing more than that 'something IS.'
Ontology is a subdiscipline of Metaphysics. Metaphysics was inaugurated by the commentators on Aristotle who collected a group of his writings that went beyond the study of physics together and called them what was 'beyond' physics.
Metaphysics traditionally is comprised of both Ontology and Epistemology.
The distinction between epistemology and ontology arises from the deep split in the Western worldview. That split fractures again by producing the dualism of logos and physus.
From the beginning of the Greek tradition the spit between physics and meta-physics was established. Thales said the most fundamental principle was Water. Anaxamander said that it was the Apeiron, or unlimited. Each early philosopher contended that the most fundamental principle had a different characteristic. This fascination with the ultimate nature of all things has extended down to present day philosophy.
The reason for studying ontology is that it tells us about the general structures of our world. We can contrast these most general structures with the structures produced by other worldviews. In this way we may be given very deep insights into our own worldview. For instance, we might be given some insight into the role of duality in our own worldview as compared with other worldviews.
First there is a distinction between the metaphysical principle (Apeiron) and some matter.
Here matter is taken generally to be a finite state of affairs of some sort.
Apeiron means the unlimited. We may chose from a whole gambit of different metaphysical principles to serve in the place of the Apeiron.
Second there is the duality between physus and logos.
Third there is a valuing of one side of the duality over and against the other by an association with the metaphysical principle.
Materialism is the result when physus is associated with the metaphysical principle and logos is considered the matter dominated by the principle.
Idealism is the result when logos is associated with the metaphysical principle and physus is considered the matter dominated by the principle.
Here the finite matters can either be logos or physus depending on ones orientation.
The distinctions between the Apeiron or the limited matters (whatever they are) AND between physus and logos are groundless.
Groundlessness means that we cannot track down and finally establish either of these distinctions.
Each distinction is like a mobius strip in that they are locally dual and globally non-dual.
Any two distinctions are like the Penrose Triangle in that they are locally coherent as mobius relations but globally they are incoherent.
The combination of the mobius strip and the penrose triangle formation is called the Kosmic Monad. A kosmic monad is a model of the grounlessness of distinctions. The kosmic monad is the model of the intersection of form and formlessness.
Groundlessness means that any different group of philosophers will draw the fundamental distinctions in a different way and will connect the resulting distinctions so formed in different ways. All these different ways of drawing and connecting distinctions forms a field of structural possibilities. The different distinctions and different possible relations drawn by different people tend to cancel each other out resulting in nihilism, i.e. no real distinctions have been drawn only apparent distinctions.
If we want to know the truth of metaphyiscis then we must look at the whole structural field. That whole structural field of possible metaphysics is known as manifestation. Manifestation is the basic referent of Being. Being as manifestation exemplifies a certain intrinsic flawed structure which has been called the Fragmentation of Being. It is that intrinsic flawed structure of Being as manifestation that is the proper study of ontology.
Reinhardt Grossman, THE EXISTENCE OF THE WORLD: An Introduction to
Ontology