[from Aulus Gellius, Attic Nights XIX 1, 14-21] A renowned Stoic philosopher drew from his satchel Book Five of the Discourses of Epictetus, edited by Arrian, writings that undoubtedly agree with those of Zeno and Chrysippus. There (translated from the Greek) we find a passage to this effect:
Impressions (which philosophers call), striking a person’s mind as soon as he perceives something within range of his senses, are not voluntary or subject to his will, they impose themselves on people’s attention almost with a will of their own. But the act of assent (which they call) which endorses these impressions is voluntary and a function of the human will. Consequently, when a frightening noise comes from heaven or in consequence of some accident, if an abrupt alarm threatens danger, or if anything else of the kind happens, the mind even of a wise man is inevitably shaken a little, blanches and recoils – not from any preconceived idea that something bad is about to happen, but because certain irrational reflexes forestall the action of the rational mind.
Instead of automatically assenting to these impressions (i.e. these frightening mental images), however (that is), our wise man spurns and rejects them, because there is nothing there that need cause him any fear. And this, they say, is how the mind of the wise man differs from the fool’s: the latter believes that impressions apparently portending pain and hardship when they strike his mind really are as they seem, so he approves (the word the Stoics use when discussing this matter) them and accepts that he should fear them as if this were self-evident. But the wise man, soon regaining his colour and composure, (does not assent), reaffirms his support of the view he’s always had about such impressions – that they are not in the least to be feared, but are only superficially and speciously frightening.
Fragments 9.9