“Suppose you cut yourself badly with a rusty knife. Given your beliefs, the thought might occur to you that you got infected. And the further thought might occur to you that you might die from this infection. At this point this is a mere impression or thought which you find yourself with. It is a disagreeable, perhaps even disconcerting, thought; that is to say, the mere thought in itself is disconcerting. The question then arises: “What is the source and nature of this disquieting character of the impression?”
According to the Stoics, there are two possibilities. The first is this: you wrongly believe that death is an evil, perhaps even a terrible evil. No wonder, then, that the mere impression that you might die is very disturbing. The second is this: you rightly believe, not that death is an evil but that it is natural to try to avoid death, and that nature means you, other things being equal, to try to avoid death. So the impression that you might die has an alarming character; it puts you on alert. This has a teleological function. It alerts you to the need to be on your guard. And, by a natural mechanism, your whole body will go into a state of alert, ready to move as needed. But the impression, though alarming, is not deeply disturbing. For, after all, you have a clear mind, and you know that there are many false alarms; and even if there is reason for alarm, you as a Stoic know that all you have to do is try to do what you can to avoid death. This is what you are meant to do. You do not actually have to avoid death. That is a matter of divine providence. So the question of whether you are going to die or not in this sense does not affect you at all. This is God’s problem, as it were.
But in the case of the person who believes that death is a terrible evil, the alarming character of the impression, which teleologically is just a signal to be on one’s guard, turns into a deeply disturbing experience, and as a consequence the whole body goes into a disturbed, perturbed, or excited state, which might affect the operation of reason.
We have to firmly remember, though this might not be so clear to the person in a deeply disturbed state, that so far we are dealing with a mere impression or thought. Naturally, as the thought may occur to you, it may also be false. After all, we have not yet found out, or made up our mind, as to whether we actually got infected. And we have not yet considered whether we should believe that one may die from this infection. So far we have just the mere thought. Now, one cannot be afraid that one might die from this infection unless one believes that one got infected and that one could die from this infection. We clearly have to distinguish between concern and fear, on the one hand, and the alarming or disturbing character of the impression, on the other hand. The wise person will be concerned, but the foolish person who believes that death is an evil will be afraid. Thus fear, according to the Stoics, is nothing but the false belief that an evil is coming, or might come, one’s way — a belief generated by assent to an impression which is deeply disturbing because one wrongly takes the situation to be an evil.
Michael Frede, A Free Will: Origins of the Notion in Ancient Thought