Republic, Plato.

The premise: set on Earth, Socrates has a dialogue with various of his friends, acquaintances,and knowledgeable Athenian personages concerning the nature of justice. He plays devils advocate more than once to help to argument along, but explores every facet of the concept in the pursuit of understanding.

Intriguing. Reading this and the Sumna Theologica at the same time has provided a nice literary synergy.