As with Book 2, Book 3 begins with a prescript identifying where Marcus composed his entries: “Written in Carnuntum,” where he was campaigning (16). The book is especially concerned with the characteristics of a good life and good man.

Marcus writes that it is necessary to “have a sense of urgency” since life is brief and the capacity for attention and understanding “will fade before we do” (16). He cites prominent figures as examples of the body’s fragility and mutability: Despite their famed works, Hippocrates, Alexander, Pompey, Julius Caesar, Heraclitus, Democritus, and Socrates all had to face death, but “nothing is empty of the gods” (17). Observing Nature as a Whole allows one to appreciate what may seem unattractive in isolation. He exhorts himself not to waste time thinking of anything other than bringing his work “to fulfillment,” making it excellent, and in a way that promotes what is good, bearing in mind “the kinship of all rational beings” (18).

Marcus’s prescriptions for living a virtuous life include allowing the god that dwells within him to “be the champion of the being you are” (18). He discusses the importance of scrutinizing the world around him and, with sound judgment, choosing only what is good.