In the course of the Republic Plato will argue that not only do moral truths, when revealed as they really are rather than contextualized into a social construct, have the force to compel a rational person’s actions; but moral truths have the force to compel the rational arrangement for the polis, which will, in turn, help promote the good behavior of its citizens. The good polis is made by the good person, his moral character intact, and the good polis, in turn, helps turn out good persons, their moral characters intact. Plato goes a very far distance in the Republic beyond both the nihilism of Thrasymachus and the social constructivism of Glaucon.
Goldstein, Rebecca. Plato at the Googleplex (p. 159). Knopf Doubleday Publishing Group. Kindle Edition.