This chapter addresses the puzzling question of why, in his Timaeus, Plato combines two very different topics: a cosmogony and account of the universe, on the one hand, and a story about the moral actions of ancient Athens, Atlantis, and Egypt, on the other. Sattler argues that the key to understanding the relation between these two parts is recognition that, in Plato’s view, they confront us with a structurally similar problem: how we are to account for the intelligibility of processes in the phenomenal world. Sattler shows that Plato no longer chooses to solve this problem by tying intelligibility to complete uniformity, as he did in the Republic, but by tying intelligibility to a rule – to norms and laws for actions in the human cultural realm, and to ratios and descriptive rules for the motions of the heavenly bodies in the natural realm. While Plato also accounts for the concerns specific to ethics and physics, the attempt to understand processes raises similar problems for him in both realms. Recurring natural catastrophes, such as floods and fires, appear as one kind of natural regularity in this Platonic account.

Part 5 revisits some earlier themes and takes some of them a bit further. Topics include: the indefinability of ‘good’; the mistake of modelling dialectic on mathematics; the relation of the form of the good to other forms; the sun-like form of the good versus the participand; the correlativity of intelligence and the good; the role of moral and intellectual education; the practicality of the rulers’ dialectic and how this makes it impossible for Socrates to produce examples; how the ‘ordinariness’ of this sort of reasoning is not at odds with Plato’s solemnity about it; how, even when writing the Republic, he could have envisaged dialectic as including investigations of a more theoretical and technical nature than those which (according to this book) characterize his ideal rulers; and, finally, the Republic’s stance of naïve realism about forms.

Part 4 addresses some questions arising. (1) Elsewhere (and, we assume, in the Republic) Plato recognizes a ‘good-making’ good – that without which all other goods lack value – which he identifies with wisdom. How does the ‘good-maker’ relate to the sun-like form? (2) The Republic recognizes a form of the good in which all good things participate. This entity cannot be straightforwardly identical with the sun-like form since the latter is interrogative whereas the former answers to the predicate of a true declarative sentence saying that something is good. How are these ‘two’ entities related? Discussion shows that the sun-like one is metaphysically prior to the participand. (3) What is the purpose of the rulers’ mathematical education? The Republic is explicit that it constitutes their induction into rationality. It neither says nor implies (so the present argument) that their ethical expertise will be couched in mathematical language, or that dialectic is responsible for a programme of grounding mathematics on ultimate metaphysical principles. (4) What is the role of the form of the good in the divine crafting of the cosmos? Does the interrogative interpretation apply here too? And is the sun-like form of the good actually a god itself?

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.

The topics of Part 1, an introductory chapter, include: the form of the good; dialectic; the possibility of Plato's 'Callipolis'; the nature of the true philosopher; the philosopher-rulers' intellectual task; the 'most important thing(s) to learn'; the virtues; the meaning of the 'longer way'; the good as a reference point for distinguishing true from false accounts of virtues and virtuous things; and the definability (or not) of the good.

It is familiar that, in Phaedo 95e ff., Plato argues that causation requires Forms and that Forms are causes. According to the standard and mainstream view, Plato’s argument for this view relies on the view that Forms are self-predicative (i.e., the Form of F is itself F) and that the cause transmits its character to the effect. The chapter demonstrates that Plato’s argument depends on neither of these views. It shows that what the argument relies on is the view that, first, Forms are essences (i.e,. the Form of F is what it is to be F), and, secondly, causation/explanation is uniform (i.e., same cause if, and only if, same effect).

The chapter considers why, and in what sense, Plato thinks that Forms are separate from sense-perceptible things. It argues that the notion of separation Plato operates with is not a purely modal notion, but rather, an essentialist notion: A is separate from B, if, and only if, What A is makes no reference to B, but what B is makes reference to A. It demonstrates that, for Plato, Forms are, in this sense, separate from sense-perceptible things, because what the primary Forms are, such as oneness or likeness, does not have to make reference to sense-perceptible things.

The chapter considers the relation, in Plato's Phaedo, between knowledge and enquiry. It argues that Plato's account of knowledge is not independent of his account of enquiry and that certain reatures of his account of knoweldge are based on his account of enquiry.