Which branch of philosophy is concerned with the nature of reality, including abstract concepts such as being?

The branch of philosophy is concerned with the nature of reality, including abstract concepts such as being, knowing, substance, cause, identity, time, and space is metaphysics. It concerns existence and the nature of things that exist. Apart from ontology (the philosophical study of being), metaphysics concerns the nature of, and relations among, the things that exist. The metaphysical idea that reality exists independently of one's mind and yet can be known is called realism.

At its heart, a metaphysical belief is one that has been derived from the Greek meta ta physika ("after the things of nature"). The initial Geeks studying metaphysics saw it as referring to an idea, doctrine, or posited reality outside of human sense perception. In modern philosophical terminology, metaphysics refers to the studies of what cannot be reached through objective studies of material reality.