Platonic literature of all countries, led the author to determine the chronological order of about twenty among the most important of the Platonic dialogues. On this basis an account of Plato's logical theories and of their development is given here for the first time. It is ascertained that the theory of ideas, generally believed to be the unique form of Plato's logic, was only a first attempt of the philosopher to settle the difficulties of the relation between Knowledge and Being and that, when past fifty, he produced a new logical system, in which he anticipated some conceptions of modern philosophy, arriving at the recognition of the substantial existence of the individual soul and substituting a classification of human notions for the intuition of divine ideas.