In this (abridged) collection of letters, Seneca presents his moral teachings, heavily influenced by the Stoic school of philosophy, to Lucilius, a young man who belongs to the same upper echelon of Roman society as Seneca. Though the letters cover a great breadth, one theme in particular permeates his thought: true happiness is achieved through inner peace and cannot be taken away by the winds of fortune.

Seneca wrote these letters as an old man and makes frequent reference to the various changes his age has brought with it. While some of these are negative—he complains of many illnesses, for example—for the most part he finds old age suits his philosophical lifestyle. His passions have withered away and he is less tempted to engage in immoral excesses and, in retirement, he can dedicate himself entirely to studying and writing. Nonetheless, he stills sees himself on the journey towards moral perfection. To help himself along this journey he engages in (and recommends) abstaining from even morally acceptable pleasures, like good food or a comfortable bed. While there is nothing wrong with such things, it is good for a man's soul because it makes him less beholden to the whims of fortune. After all, a person who is rich one day might find himself poor and homeless the next. The Stoic knows, though, that no matter what one's external circumstances are, true happiness comes from within.

Seneca's understanding of philosophy is thoroughly practical. While he engages, from time to time, in the abstract theorizing that is commonly associated with it, he views it as secondary to the real purpose of philosophy: learning how to live well. Philosophy is not a pastime or hobby; it is a way of life that must permeate through everything a man does. Though it may seem paradoxical for a philosopher to have an anti-intellectual streak in him, Seneca is explicitly dubious about the value of the liberal arts, like history and literary analysis. He asks what value those things can have—when will one ever need to know about the stylistic similarities between Homer and Virgil? Such studies, certainly, do not teach a man how to live better and, thus, they are only barely better than spending all of one's time exercising or seeing public shows (activities which he also condemns). Their only possible value is that they cultivate certain good intellectual habits in the mind and thus prepare one for the study of philosophy.

However, Seneca is not entirely uncritical of philosophy or, at least, what passes for philosophy. He is immensely frustrated by those who want to reduce philosophy to cleverly arranging words into meaningless and absurd syllogisms. Such an art is not really philosophy—the love of wisdom—but philology—the love of words. Instead, he prefers to argue through example. There is no need to give a logical argument against drunkenness: Just look at how foolishly drunk people act. It should be obvious to anyone, he says, that such behavior is not fitting for a wise man.