Being has been the central and dominating concept of twenty-five hundred years of Western philosophy. Ontology is the branch of metaphysics that concerns itself with the nature of being. In this sense it is at the core of metaphysics and is the central problem of philosophy. As Aristotle has put it, “That which now and always has been asked and now and always perplexes us is the question: ‘what is being?’” Aristotle did not use the word ontology, but called the study of being, “First Philosophy”. The special sciences study being “in some way”, το ον πως, whereas “first philosophy” studies being as being, being qua being, being as it is in itself; το ον η ον, or as Plato had put it before him, the being of being, το όντως ον. First philosophy, therefore, is a science which is prior to all the other sciences and is therefore termed the science of sciences, επιστήμη επιστημών.