The theory of the Form of the Good in the Republic is truly and coherently the centerpiece of the canonical Platonism of the middle dialogues, the centerpiece of Plato's metaphysics, epistemology, ethics and politics, and even his theory of love and art. In this theory of the Form of the Good Plato was truly the first grand philosophical synthesiser. If to achieve such a grand synthesis he had to employ a few unholy combinations, such as the combination of reality, goodness, and self-predication, he may perhaps be forgiven - at least if he is understood.
Santas, Gerasimos X., "The Form of the Good in Plato's Republic" (1977). The Society for Ancient Greek Philosophy Newsletter. 85.