ontology ：In philosophy, ontology, is the most fundamental branch of metaphysics. It is the study of being or existence as well as the basic categories thereof. It has strong implications for the conceptions of reality.
reality：Reality in everyday usage means "everything that exists." The term "Reality," in its most liberal sense, includes everything that is, whether or not it is observable, accessible or understandable by science, philosophy, theology or any other system of analysis. Reality in this sense may include both being and nothingness, whereas "existence" is often restricted to being.
In the strict sense of European-German philosophy, in the tradition of Ludwig Wittgenstein, there are levels or gradation to the nature and conception of reality. These levels include, from the most subjective to the most rigorous:
noumenon:In the philosophy of Immanuel Kant, a noumenon or thing in itself (German Ding an sich) is an unknowable, undescribable reality that, in some way, lies "behind" observed phenomena. Noumena, plural, are sometimes spoken of, though the very notion of individuating items in "the noumenal world" is problematic, since the very notions of number and individuality are among the categories of understanding -- so that individuality itself is a noumenon. "Phenomenon" is another technical term in Kant's philosophy, meaning the world as experienced.
One of the most difficult problems for Kant's philosophy is explaining the relationship between the noumenal and phenomenal worlds. On Kant's view as expressed in his Critique of Pure Reason, reality is structured by so-called "concepts of the understanding," or innate categories that the mind brings to make sense of raw unstructured experience. Since causality and number are among these categories, it is problematic to say that there are "many" noumena that individually "cause" us to have perceptions of phenomena. But if the noumenal is not the cause of the phenomenal, then what is the relationship? The answer is that the noumenal and phenomenal coexist simultaneously; neither can be said to cause or be caused by the other.
It can be said that on Kant's view the noumenal is radically unknowable. Whatever concept we might want to use to categorize some noumenon or noumena, that is only a way of categorizing phenomena, so that the act of knowing a noumenon must itself be defined by a noumenon, a situation that is unresolvable.