The aim of Part 3 is to make sense of Plato’s succinct ontological assertion that the sun-like good is source of the being and reality (or essence) of ‘the other’ forms. The text rules out equating this with the other forms’ participation in the form of the good. Two positive interpretations are put forward, one whereby ‘the other' forms are forms of virtues, the other whereby they are ethically neutral types such as returning a borrowed item to its owner. Both interpretations are closely grounded on Plato’s precise wording of his ontological claim. And, unlike various current interpretations, both allow for a measure of continuity between Socratic argument in earlier dialogues and dialectic in the Republic. Other interpretations are considered and rejected: the idea that the form generates the other forms by self-diffusion; the perfectionist approach that identifies the form of the good with the perfection or ideality as such common to all specific forms; and the approach that sees the form of the good as in some sense the system of other forms.