On the Origins of Emotional Responses
One of the hardest concepts in Stoic thought with which we may grapple is, I think, the origins of what feel like "reflexive" emotions. An impression appears to our senses, and it seems to provoke an immediate response. Stoics say that this "pre-emotion" is natural - our minds are being offered the option of agreeing to admit an emotion. If we assent to it, we "let the emotion in," so to speak, and the emotion takes hold of us. But if we do not give it our assent, then it cannot affect us. The "pre-emotion" is all there is - a gentle nudge in the direction of an emotion, not strong enough to actually move us in that direction unless we allow it to do so.
Here are some thoughts from Margaret R. Graver's Stoicism and Emotion on the subject:
"Considering the kinds of phenomena mentioned as examples, one may find it slightly odd that Seneca chooses the expression `beginnings preliminary to emotion' (principia proludentia adfectibus). Most of the examples given are not in fact preliminaries to anything. One does not go on from a conditioned reflex, a sympathy response, or an aesthetic response to experience emotion for real: in these cases, the `beginning' is the entirety of what occurs. Again, though, the choice of expressions is comprehensible if one bears in mind that the purpose of the entire discussion is to give the closest possible analysis of the process by which anger, real anger, is generated. Foremost in Seneca's mind is the distinction between two events: first, the feeling which may accompany a fully conceptualized impression that taking revenge is now appropriate; and second, the endorsing of that impression, in which, consequent upon opportunity, revenge taking actually begins. Since the endorsing, if it takes place at all, must have been preceded by the impression, it is reasonable to describe the feeling connected with the impression alone as `preliminary,' even though there will be cases in which no assent is ever given. We do not have to assume that Seneca was familiar with the term `pre-emotion' (propatheia), which we know to have been used at Alexandria. More important is that he has in mind the same notion as must have suggested that term: that whether or not the unassented feeling will ever become an emotion, it certainly is not one yet."
Margaret R. Graver. Stoicism and Emotion (p. 98). Kindle Edition.