Investigating Plato’s ontological as well as epistemological status in each of his dialogues, this book is going to challenge the current theories of Plato’s development and suggest a new one. Regarding the relation of Plato’s early and middle period dialogues, scholars have been divided to two opposing groups: unitarists and developmentalists. While developmentalists try to prove that there are some noticeable and even fundamental differences between Plato’s early and middle period dialogues, the unitarists assert that there is no essential difference (...) in there. The main goal of the first two chapters is to suggest that some of Plato’s ontological as well as epistemological principles change, both radically and fundamentally, between the early and middle period dialogues. Though this is a kind of strengthening the developmentalistic approach corresponding the relation of the early and middle period dialogues, based on the fact that what is to be proved here is a essential development in Plato’ ontology and his epistemology, by expanding the grounds of development to the ontological and epistemological principles, it hints to a more profound development. The fact that the bipolar and split knowledge and being of the early period dialogues give way to the tripartite and bound knowledge and benig of the middle period dialogues indicates the development of the notions of being and knowledge in Plato’s philosophy before the dialogues of the middle period. The first chapter entitled “Plato’s Onto-Epistemological Principles in the Early Dialogues” tries to draw out six principles out of Plato’s early dialogues specially Euthyphro, Laches, Charmides, Hippias Major and Euthydemus. We discuss that these principles present kind of a bipolar as well as split ontology and epistemology. The second chapter, “Revision of First Socrates’ Principles in the Middle Period Dialogues”, aims to argue that the onto-epistemological principles of the early dialogues are being radically changed in three dialogues of Meno, Phaedo and Republic in the middle period dialogues. Not only the bipolar ontology and epistemology of the early dialogues give place to a tripartite ontology and epistemology but also their split being and knowledge are inclined to be replaced by bound being and knowledge. Our next step in this book is to suggest a new approach to Plato’s theory of being in Republic V and Sophist based on the notion of difference and the being of a copy. To understand Plato’s ontology in these two dialogues we are going to suggest a theory we call Pollachos Esti; a name we took from Aristotle’s pollachos legetai both to remind the similarities of the two structures and to reach a consistent view of Plato’s ontology. Based on this theory, when Plato says that something both is and is not, he is applying difference on being which is interpreted here as saying, borrowing Aristotle’s terminology, 'is is (esti) in different senses'. I hope this paper can show how Pollachos Esti can bring forth not only a new approach to Plato’s ontology in Sophist and Republic but also a different approach to being in general. Thence, chapter three, “Pollachos Esti; Plato’s Ontology in Sophist and Republic”, intends to discuss that i) the theories of ‘being as difference’ and ‘being of a copy’, considered together in what we call the theory of pollachos esti, can well be compared to the structure of pollachos legetai in Aristotle when it is attached to the theories of pros hen and substance; and ii) the ontology of Republic V-VII is based on this theory and is, thus, almost the same as the ontology of Sophist. Investigating the most famous chronologies of the last 150 years from Campbell on, the fourth chapter, “The Standard Chronology of the Dialogues”, is to argue that all of them have a somewhat fix and dogmatic arrangement of Plato’s dialogues in which Meno, Phaedo and Republic are located after some early dialogues and before Theaetetus and Parmenides, on the one hand, and all the so-called late period dialogues after Theaetetus and Parmenides on the other hand. It is also reminded that all that the stylometric evidences can show is the lateness and homogeneity of the late period dialogues and, thence, nothing about the relation between dialogues like Theaetetus, Parmenides and Republic. The standard chronology is the subject of many criticisms some of which are discussed in our fifth chapter, “Objections against the Standard Chronology”, in three groups. While the first group of objections criticizes the place of the middle period dialogues immediately after the early ones, the second group attacks the place of late dialogues after the middle ones. The third group includes objections against the place of Parmenides in the standard chronology and tries to show that it cannot be considered after the middle period dialogues. The efforts of the first five chapters lead to a new theory of Plato’s ontological as well as epistemological development in an onto-epistemological chronology of his dialogues in our sixth chapter, “An Onto-Epistemological Chronology of Plato’s dialogues”. Instead of three periods, this chronology includes four waves of dialogues, Socratic wave, ontological wave, epistemological wave and political wave, in which all the so-called middle and late period dialogues are to be interpreted based on the problems presented in Parmenides I. The main changes we suggest in the standard chronology include firstly that Theaetetus and Parmenides I must be posited before Meno and Phaedo and, secondly, Republic must be posited after Sophist. Based on this arrangement, we can find Philosophos, Plato’s promised but unwritten dialogue, inside Republic. (shrink)