This paper is the brief introduction with which I chaired the medieval session of the Conference on the history of ontology. It proposes some simple and general considerations, which aim to provide an absolutely essential historical-conceptual outline of medieval metaphysical thought, and to express a way of envisaging its unity. My thesis is that medieval ontology is indifferently theological (theocentric) in character, and that one of the tasks of the historian is to understand how an “Aristotelian” metaphysics could have been (...) formed in correspondence with a theological and Christian knowledge, and through what theoretical and conceptual transformations this Aristotelian metaphysics could have constituted the instrument of elaboration and justification of such a knowledge (and its historical evolutions). (shrink)