One of the hardest things for Stoics to explain is, and always has been, the fact that most of us seem to experience emotions to which we do NOT assent. Margaret R. Graver's Stoicism and Emotion addresses this issue in a famous story from Aulus Gellius:
"The anthologist Aulus Gellius tells a story purporting to be from his own experience aboard a ship at sea. A storm arises; the passengers on deck are facing imminent destruction. One passenger, however, is known to be a Stoic philosopher, and the inquisitive Gellius finds himself watching this man in spite of his own extremity. The philosopher does not scream or cry out as others are doing, yet his appearance is hardly unmoved: his complexion is pale, his hands tremble, and his expression is one of alarm. After the storm passes, Gellius seeks an explanation. Should not philosophy have guarded against such feelings?
The philosopher is not the least bit embarrassed. "Since you wish to know," he replies, "hear what our forebears, the founders of the Stoic sect, believed concerning that alarm which is brief, but necessary and natural. Better yet, read it, for reading will make it easier to believe and also to remember." He then produces from his satchel a volume of Epictetus's Discourses and points out the following words:
'Mental `impressions,' through which a person's mind is struck by the initial aspect of some circumstance impinging on the mind, are not voluntary or a matter of choice, but force themselves upon one's awareness by a kind of power of their own. But the `assents' through which those same impressions are cognized are voluntary and happen by one's own choice. That is why, when some terrifying sound occurs, either from the sky or from the collapse of a building or as the sudden herald of some danger, even the wise person's mind necessarily responds and is contracted and grows pale for a little while, not because he opines that something evil is at hand, but by certain rapid and unplanned movements antecedent to the office of intellect and reason. Shortly, however, the wise person in that situation `withholds assent' from those terrifying mental impressions; he spurns and rejects them and does not think that there is anything in them which he should fear.
And they say that between the mind of the wise person and that of the nonwise there is this difference, that the nonwise person thinks that the kinds of things which when they first struck his mind impressed him as scary or harsh really are that way, and `adds belief,' endorsing those same beginnings as things rightly to be feared; but the wise person, although he experiences a brief and superficial response in color and expression, does not `assent,' but maintains the state and strength of his opinion which he has always had about impressions of that kind, namely, that they are not at all to be feared but alarm us by false appearance and empty fright.'
Gellius declares himself satisfied that the passage he has thus translated accords fully with the writings of Zeno and Chrysippus. In his judgment, then, the philosopher's excuse is textbook Stoicism. Fear, like every emotion, is to be eliminated-but what the pale and trembling passenger experienced was not an instance of fear.
In the passage thus recorded, Epictetus examines the process by which emotions are generated, using as his paradigm the emotion of fear. That process begins involuntarily, with a mental impression (phantasia) in which some circumstance-say, the rumble of a building which is about to collapse-strikes the mind as an impending evil meriting fear. When such impressions occur, says Epictetus, one necessarily experiences certain sensations, even if one is wise. The terms used to refer to these sensations alternate between inner experience and visible physiological changes: the mind itself "is contracted and grows pale," and there is also a change in "color and expression." However, the morally significant question is not what these sensations are, but whether one assents to the view that has presented itself, that the thing that seems about to happen really is "scary or harsh." One who is wise will not assent and thus remains free of fear; in his case, there is only a "brief and superficial response."
The concession Epictetus makes here has two far-reaching implications for his presentation of Stoic views on emotion. First, it implies that the concept of `emotion' (pathos), as understood by Stoics, is delimited by something other than changes of color, expression, and other observable signs of arousal. A person could undergo some verifiable physiological alteration, in the presence of the kinds of stimuli that frequently trigger emotion, and yet not have the emotion, if he or she does not also believe certain things. The Stoic claims about the voluntariness of the pathē would then not apply to all phenomena that might loosely be called affective, but only to a subset of them. This clarification can make a great difference in how the school's position on the pathē is mapped onto lived experience. I feel a flash of irritation at my spouse who has eaten the last plum: is this anger? According to this fragment of Epictetus, a Stoic can say that unless further conditions are met, what I feel is not anger at all. It is not even a trivial case of emotion, and the claims made about emotion do not apply to it.
A second, and related, implication concerns the Stoics' normative conception of a life in which emotions, properly so called, have ceased to occur. We have seen that the apatheia or `impassivity' which comes with wisdom does not exclude what the Stoics called eupatheiai, `well-reasoned' upliftings, reachings, or withdrawings of the psyche in relation to perceived goods and evils. But eupathic response, in that it is well reasoned, is concerned always with integral goods and evils; that is, with a person's own character and actions. For external objects the normative human being does not have any affective response at all, since he or she does not recognize these as either good or evil. We now learn, however, that the wise person does still feel something in connection with kinds of things that are the usual objects of the pat he. The normative human condition does not preclude having an impression that the crash of thunder or the crumbling plaster indicates an evil in prospect. That the wise person still has such impressions, and with them some trembling, pallor, or `contraction,' implies that he or she still has the capacity to respond to external objects just as the ordinary person does. It is just that that capacity is no longer exercised."
Margaret R. Graver. Stoicism and Emotion (pp. 85-87). Kindle Edition.