Plato's Moral Realism: The Discovery of the Presuppositions of Ethics

Surveying many of Plato's dialogues from the early, middle, and late
periods, prominent philosopher John M. Rist shows how Plato gradually
realized the need for metaphysics to support his ethical position
and that a rigorous ethics required a secure metaphysics grounded in
universal values.
Plato came to realize that his earlier attempts to construct the relevant
metaphysics, culminating in the Republic, were incomplete and
his argumentation was insufficiently rigorous. Rist explains Plato's ongoing
refinement of the theory of Forms and his hesitant attempts to
relate claims about Forms to ideas about a divine mind (or god), which
could offer an account of a transcendent reality as not only a formal
and final cause of cosmic goodness and providence, but also an efficient
cause.
Rist concludes the book by considering what more would be needed
to complete Plato's theory without making damaging compromises to
the basic principles of his metaphysics of morals. He sketches how Plato
might reply to various contemporary approaches to moral reasoning
and especially moral obligation.