This dissertation examines the idea of the good in the Republic of Plato, in the famous images of the sun (505a-509b), the line (509d-511e) and the cave (514a-521b; 531c-535a). After an introduction that justifies the importance of the subject and brings the central hypothesis of this study, it is described the context in which it is presented the idea of the good, as the fundament of the capital proposition of the dialogue, the philosophers’ government. In the following chapter, the third one, it is analyzed the use of images, conducted by Socrates to convince his interlocutors of the necessity of this maximum knowledge and the way of knowing it, the dialects that renounce images. In this third chapter, it is yet studied the metaphor and the analogy related to the theory of ideas, to understand the apparent paradox of condemning images using them. The aim of the fourth chapter is to comprehend the analogy of the sun, which represents the causal function of the good and indicates its ontological and epistemological condition, beyond being and essence. The fifth chapter studies the image of the line, to identify the position of the good, as the non hypothetical first principle of everything, in the general structure of reality, indicating the state of soul that knows it, the noesis. The sixth chapter studies the allegory of the cave, its pedagogical aspect and the form of dialectical knowledge able to achieve the idea of the good, demonstrating the practical consequence, ethical and political, of this knowledge.