Metaphysics, in its classical version, deals with the basic understanding of reality i.e. things as concretely existing; it tries to give the answers to the following questions: What is being? Which are the transcendental proprieties of being and how to understand them? What is the structure of being, founding the problem of the causes and casuality as such? How to understand the analogy of being and connected with it the analogy of knowledge, in the ultimate perspective of the existence of God? The solutions of these problems go through the understanding of the interpretations of being given by Aristotle, St. Thomas Aquinas and modern thinkers.