The Lunyu, usually rendered Analects and more literally something like selected sayings, is not a treatise and does not argue in sequence. It is a collection of short units, most of them a sentence or two: a question from a disciple and a reply, a judgement on a historical figure, an observation about someone's character, occasionally a note on how the Master stood, ate or dressed. The twenty books are named from their opening words and have no thematic organisation worth the name.
Four terms carry most of the weight. Ren, written with the graphs for person and two, is the humaneness that consists in treating others as fully persons; Confucius refuses to define it abstractly and gives different answers to different questioners, including the negative form of the golden rule, not imposing on others what you would not want yourself. Li is ritual propriety, covering everything from sacrifice to how one greets a guest, and the claim is that outward form and inward disposition make each other rather than one merely expressing the other. Junzi, once meaning a ruler's son, is redefined as the man of cultivated character, which detaches worth from birth and is among the book's most consequential moves. Zhengming, the rectification of names, holds that disorder begins when words no longer match realities: a ruler who is not behaving as a ruler should not be called one.
The tone is worth noting. The Confucius of the Analects is often blunt, sometimes funny, capable of irritation and of admitting failure, and he insists he transmits rather than creates. It is a portrait, not a system.