  Zabarella’s introductory treatise on the nature of logic, De natura logicae, is basic to his teaching in logic. He defines logic as being neither a science nor an art, but, in keeping with the traditional meaning of the word organon, just an instrument (instrumentum) of the arts and sciences. As an instrumental discipline it furnishes a useful tool of inquiry for all the arts and sciences. Logic does not have a real subject of its own, but deals with concepts, which stand for real beings. In this it is comparable to grammar. The difference between grammar and logic is that the former is concerned with the perfect verbal expression of concepts, and hence is a linguistic discipline, while the latter invents second notions (notiones secundae) or second intentions, that are able to create order among concepts. Therefore logic serves to recognize the truth and distinguish it from falsehood in every instance. Logic is thus a rational discipline (disciplina rationalis) that is not itself philosophy, but springs from philosophy and is devoted to philosophical ends (Vasoli 2011).