Each of the three disciplines - assent, desire and action, concerns itself with a realm of knowledge or 'science'. The Discipline of Assent is associated with the science of logic. Logic is the tool we use to interrogate our impressions about the world and to formulate correct ones. The Discipline of Desire is associated with Physics, since Physics is an exploration of the laws of Nature - laws which dictate behavior in the world around us that occurs independent of our will and control. The ancient Stoic philosophers associated the Discipline of Action with Ethics - the science - in Greek epistime - of how to act and interact with the world, and Virtue, which Stoic philosophers defined as the techne or knowlege of the correct way to live.
The French historian of philosophy Pierre Hadot describes the Discipline of Action as "all the acts and movements which respond to the requirements of integral human nature... the faculty of growth, of sensation, and of reason." Hadot argues persuasively that the chief ethical cause for the Stoic Marcus Aurelius, for example, is serving the invisible city of reasoning beings that make up the world. This is defined as the virtue of dikaisoyne or 'justice'. %p The following passage from Marcus Aurelius highlights his goal, "Impassivity (ataraxia) with regard to the events, [Discipline of Desire] brought about by the exterior cause. Justice (dikaiosyne) in the actions brought about by the cause that is within you. In other words, let your impulse to act and your action have as their goal the service of the human community, because that, for you, is in conformity with your nature" (IX, 31).
Justice is one of four classical virtues (again, knowledge about the correct way to live and act, thus at the heart of the Discipline of Action) found already in Plato and later alive in the Stoic tradition. The other threecardinal virtues are sophia (wisdom/truth), agothos (strength/courage) and eusebes (piety/temperance).
Stoic written exercises associated with action tend to look like the Marcus Aurelius passage cited above. The exercise reminds the practicioner of the virtue of justice, the importance of acting for the good of the human community. Other times, Discipline of Action exercises trigger reflections on all four virtues. For example, Marcus Aurelius writes, "And how is he [man] to do this [live correctly]? By having principles to govern his impulses and actions. What are these principles? Those of good and evil - the belief that nothing is good for a human being which does not make him just, self-controlled, brave, and free: and nothing evil which does not make him the opposite of those." (VIII, 1).
All three disciplines are ultimately connected. By reflecting on our impressions and logic, we can more clearly determine how we ought to act. Explains Hadot, "The discipline of action, like the other disciplines in the domains in which they are exercised, will therefore begin by imposing the norms of reason and reflection upon human activity." Hadot cites Marcus' Meditation from Book XII, 20: "In the first place, nothing at random, and nothing that is not related to some goal. Second: do not relate your actions to anything other than a goal which may serve the human community." Hadot concludes, "The vice which is opposed to the discipline of action is thus frivolity (eikaiotes). It is the opposite to that seriousness or gravity with which all human actions should be accomplished. This human frivolity or lack of reflection does not know how to submit to the discipline of action; it is the agitation of a jumping jack, a puppet or a top."
What about when stuff goes wrong?
The Stoics recognize that our actions will not always lead to the outcomes that we hope for. On the one hand - and many Stoic Penknife exercises deal with this question - the challenge afforded by failures, setbacks, disappointments, surprises, is an opportunity for growth. On other hand, they are an opportunity to practice the Discipline of Desire. Writes Hadot, "The Stoics do not only say, 'I don't know whether my action will succeed.' Rather, they also say, 'Since I don't know in advance what the results of my actions will be, and what Destiny has in store for me, I have to make such-and-such a decision in accordance with probability and rational estimate, without any absolute certainty that I am making the right choice or doing the right thing." He later, once again, quotes Marcus Aurelius, who writes, "People can perfectly well prevent me from carrying out such-and-such an action. Thanks, however to action 'with a reserve clause' and to 'turning obstacles upside down', there can be no obstacle to my intention, nor to my disposition. For my thought can 'turn upside down' everything that presents an obstacle to my action, and transform the obstacle into an object toward which my impulse to act ought preferably to tend. That which impeded action thus becomes profitable to action, and that which blocked the road allows me to advance along the road" (V, 20, 2).