The City of the Sun is a philosophical work by Tommaso Campanella. The format of the book is based on dialogue. The conversation between a Grandmaster and an Italian naval captain allows Campanella to articulate his theories and ideas on various philosophical questions. The most prominent theme of the text is the concept of Utopia. In the dialogue, the Grandmaster details a prophetic vision in which a city or empire will establish a theocratic society based on universal codes of morality.

Another prominent theme in The City of the Sun is astrology. In the book, the Grandmaster uses astrological references to articulate his prophetic visions. Astrology was a major component of pagan beliefs from multiple cultures around the world. Astrological symbolism is also used by esoteric philosophers in their teachings and applications.

The other major theme in Campanella's work is the concept of conquest. In the text, the Grandmaster believes that a theocratic society could be established in the New World led by the Spanish monarchy. This vision is in line with the actual founding of the Americas, which was initiated by the Spanish Crown, and led to the mass colonization of what is now North, Central and South America. The New World was initially used by the Spaniards to extend their Roman Catholic-based empire and, later, used as a refuge by Puritans to create a theocratic Utopia, away from the persecutions they faced back in England.

Another theme is the examination of a city's social structure. In the book, a mythical city is described in terms of physical structure and architecture, but also outlines the social structures that all citizens should live by. Campanella's utopian vision explores the concept of an egalitarian society, where all men are created equal. In essence, the philosophical theory of Campanella tries to connect theocracy-based morality and man-made legal systems in order to portray a perfect society.