Within the development of medieval philosophical thought emerges the key personality of John Duns Scotus. We reconstruct the basic outlines of his philosophical thought, as distinguished from the more strictly theological, focusing especially on his metaphysics, seen in light of the milestones of his thought: the distinction between theology and philosophy, the relationship between the goal and desire, the demonstration of the existence of God and the affirmation of the material subject as a positive reality. We give particular importance to the concept of being as the key to understanding the uniqueness of the ontology of Scotus, in direct correlation with the similarity between the substances.