The main aim of Part 2 is to explain how the form of the good gives rise to knowledge of forms, the forms in question being of virtues and virtue-related things. This ramifies into discussions of dialectic and mathematics, the ambiguous property 'clearness' (saphēneia), hypotheses, and the non-hypothetical principle. It is proposed that the form of the good is interrogative. This position is defended against philosophical and textual objections, and argued to be preferable to alternatives. There is discussion of why Plato excludes the use of diagrams from dialectic and whether he can allow input from experience. The role of context in the rulers' dialectic is explained, and becomes the basis for explaining why Plato's treatment of dialectic in the Republic remains at the level of a sketch. There is an exploration of the difference between true philosophers and sight-lovers, and of the criteria and scope of 'good' in dialectic. This last discussion encounters the classic problem of the connection between Plato's 'justice in the soul' and just conduct as ordinarily recognized, and a solution to this problem is proposed.

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.