6 ideas
The virtuous city is structured like a healthy body, with each part performing its function and ranked by its closeness to the ruling organ, the way organs serve the heart. Order flows downward: the ruler serves no one, the middle ranks both serve and command, and the lowest ranks only serve. Political health is a matter of correct functional subordination, not equality of parts.
The cosmos descends from the First Cause through a graded chain of intellects and spheres, each dependent on the one above it. The ideal city is legitimate because it copies this same ranked order, with a single first ruler whose authority grounds every subordinate rank. Political design is therefore derived from metaphysics, not invented by agreement.
The ideal ruler's rational faculty is joined to the Active Intellect, so he receives truth as a philosopher. His imaginative faculty is joined to it too, so he receives the same truth as prophecy. He needs no teacher, because he apprehends directly what others must be taught, and this is what fits him to legislate. If no single person has all the needed qualities, rule can pass to a partnership of people who hold them between them.
Most people cannot grasp demonstrative truths about first principles and happiness. So the ruler presents them through images and symbols suited to each nation's imagination. Different religions can therefore be valid representations of one philosophical truth, and people are ranked by whether they understand the truth itself or only its likenesses.
Non-virtuous cities fail in distinct ways. The ignorant city never knows true happiness and pursues wealth, pleasure, honor, conquest or bare survival. The immoral city knows the good but acts against it. The errant city holds false beliefs about God and happiness. The wayward city once was virtuous but has been corrupted. Each diagnosis pairs a failure of knowledge or of will with the false aim the citizens adopt instead.
True happiness is the soul's freedom from matter, reached by actualizing the intellect through knowledge and virtuous action. Souls in the virtuous city attain lasting bliss after death. Souls in immoral or errant cities that knew the good but refused it persist in misery. Souls in ignorant cities that never grasped it simply dissolve with their bodies. Fate after death depends on what the intellect came to know.