In Pierre Hadot's The Inner Citadel, he analyzes some of what is meant by "Virtue" and how it leads to joy . . .
Hadot believes that he detects a threefold structure repeated throughout the Meditations of Marcus Aurelius, derived from Epictetus, and accounts for it as "an internal necessity, in the sense that there can be neither more nor fewer than three exercise-themes for the philosopher, because there can be neither more nor fewer than three acts of the soul."
The Greek word translated as "Virtue" is "Aretē." Aretē originally denoted pure excellence in an aristocratic warrior-elite sense, rather than moral excellence, but over time the sense had shifted somewhat. "We may suppose that this ideal of excellence and value always remained present in the mind of the philosophers. For the Stoics, aretē is absolute value, based no longer on warrior nobility, but on the nobility of the soul represented by the purity of our intentions." Virtue is one, in Stoic thought, yet may be thought of as consisting in the four "cardinal virtues," all of which, Hadot notes, imply one another, creating this unitarian view of Virtue.
Hadot notes that Marcus "often summarizes the three disciplines - of assent, of desire, and of action - by making the names of the virtues correspond to them. Thus the discipline of assent takes on the name of the virtue of 'truth;' the discipline of desire acquires the name of the virtue of 'temperance;' and the discipline of action, that of the virtue of 'justice.'" (Substituting "trutt" for the usual phrasing, "prudence," is not surprising, notes Hadot, since Plato uses "truth" for "prudence" in The Republic). There is a lengthy passage of the Meditations (IX, 1) cited to show Marcus following this division, in which "it is easy to recognize the three disciplines: that of action, which ordains that people should help one another; that of assent, which consists in distinguishing the true from the false; and that of desire, which consists in accepting the lot which universal Nature has reserved for us. To these three disciplines correspond three virtues. In the discipline of action, we must respect the value hierarchy of people and things, and thus act in accordance with justice. According to the discipline of assent, our discourse must be true, and the virtue particular to this discipline is truth. He who knowingly lies commits a twofold sin: in the area of assent, since his discourse is not true, and in the area of action, since he is committing an injustice with regard to other people. As for the person who lies involuntarily - in other words, who deceived himself - it is because he has not succeeded in criticizing his judgments and in becoming the master of his assent that he is no longer capable of distinguishing the true from the false. Finally, in the discipline of desire, we must desire only that which universal Nature wants, and we must not desire pleasures or flee sufferings. This discipline is characterized by temperance."
According to Hadot's analysis, then, "Nature appears to us in three aspects. She is the principle of attraction, which urges human beings to help one another and to practice justice, and is therefore the basis of justice. She is also the basis of truth; that is to say, the principle which founds the order of discourse, and the necessary relationship which must exist between beings and the true attributes that are said about them. To speak falsely, whether voluntarily or involuntarily, is therefore to be in disaccord with the order of the world. Finally, universal Nature, since she is indifferent to indifferent things, is the basis of temperance, in other words of that virtue which, instead of desiring pleasure, wants to consent to the will of universal Nature."
"Marcus here portrays universal Nature as the most ancient and august of goddesses, in such a way that any lapse with regard to the virtues - justice, truth, and temperance - of which this goddess is the model and the principle, is an impiety. The Stoics traditionally identified God, Nature, Truth, Destiny, and Zeus."
"In Marcus' view, these three disciplines and virtues bring into the soul the only true joy which exists in the world, since they place the soul in possession of all that is necessary: the one absolute value." "Joy, then, is the sign of an action's perfection." "Unlike Epicurean pleasure, Stoic joy is not the motive and end of moral action: rather, virtue is its own reward. Virtue seeks nothing above and beyond itself; instead, for the Stoics, joy, like Aristotelian pleasure, comes along as an extra surplus in addition to action in conformity with nature . . ." "Such joy is not, moreover, an irrational passion, because it is in conformity with reason."