There are five explicit references to the concept of acting “with a reserve clause” in The Meditations of Marcus Aurelius, which he obviously employs as a kind of technical term. This isn’t very clear in some of the English translations available. The concept of action “with a reserve clause” (hupexhairesis) is probably closely-related to the concept of “moral choice” (prohairesis) and the definition of the good as “worthy of being chosen” (haireton) in accord with reason and nature.
The concept of the “reserve clause” (exceptio in Latin) can also be found in Seneca and Epictetus, and to some extent in Cicero’s writings on Stoicism. For example, Epictetus tells his students:
For can you find me a single man who cares how he does what he does, and is interested, not in what he can get, but in the manner of his own actions? Who, when he is walking around, is interested in his own actions? Who, when he is deliberating, is interested in the deliberation itself, and not in getting what he is planning to get? (Discourses, 2.16.15)
The basic idea is that Stoics must act in the world, although the good of their own soul (wisdom and virtue) is the chief goal in life, external and bodily things, despite being classed as “indifferent” with regard to our ultimate wellbeing, are to be “selected” or “rejected” in a somewhat detached manner, insofar it is natural and rational to either get or avoid them. In other words, we should pursue external goals with the caveat: “Fate permitting.” There’s a good description of this in the New Testament:
Now listen, you who say, “Today or tomorrow we will go to this or that city, spend a year there, carry on business and make money.” Why, you do not even know what will happen tomorrow. What is your life? You are a mist that appears for a little while and then vanishes. Instead, you ought to say, “If it is the Lord’s will, we will live and do this or that.” (James, 4:13-15)
Hence, Christians used to write “D.V.” or “Deo Volente” (God willing, in Latin) at the end of letters. However, Marcus makes it explicit that he’s deriving the concept, in part, from Epictetus, whom he quotes.
Hear Epictetus: No one can rob us of our free choice. We must, says he, hit upon the true science of assent and in the sphere of our impulses pay good heed that they are with a “reserve clause”; that they have in view our neighbour’s welfare; that they are proportionate to a thing’s value. And we must abstain wholly from inordinate desire and show avoidance in none of the things that are not in our control. (Meditations, 11.37-38)
The Stoics believed the mind was composed of a physical substance like a subtle form of fire, and Marcus describes its ability to adapt to external events, through the “reserve clause”, as resembling an all-consuming fire.
That which holds the mastery within us, when it is in accordance with Nature, is so disposed towards what befalls, that it can always adapt itself with ease to what is possible and granted us. For it is wedded to no definite material, but in the pursuit of its aims it works with a “reserve clause”; it converts into material for itself any obstacle that it meets with, just as fire when it gets the mastery of what is thrown upon it. (Meditations, 4.1)
What stands in the way becomes the way:
Though a man may in some sort hinder my activity, yet on my own voluntary impulses and mental attitude no fetters can be put because of the “reserve clause” and their ability to adapt to circumstances. For everything that stands in the way of its activity is adapted and transmuted by the mind into furtherance of it, and that which is a check on this action is converted into a help to it, and that which is a hindrance in our path goes but to make it easier. (Meditations, 5.20)
The “reserve clause” is a way of overcoming emotional pain and as the perfect Sage cannot, by definition, be happy (eudaimon) if he is distressed, then he must act at all times according to this rule.
Try persuasion first, but even though men would say to you not to, act when the principles of justice direct you to. If anyone one should obstruct you by force, take refuge in being contented and without emotional pain, and use the obstacle for the display of some other virtue. Remember that the impulse you had was with an “reserve clause”, and your aim was not to do the impossible. (Meditations, 6.50)
However, he also seems to refer this concept to the image of the “sphere” of the presocratic philosopher Empedocles, which he mentions three times in The Meditations.
If your impulse is without an “reserve clause”, failure at once becomes an evil to you as a rational creature. But once you accept that universal necessity, you cannot suffer harm nor even be thwarted. Indeed, nobody else can thwart the inner purposes of the mind. For it no fire can touch, nor steel, nor tyrant, nor public censure, nor anything whatsoever: a sphere once formed continues round and true. (Meditations, 8.41)