The Concept of "Assent"
In Margaret R. Graver's Stoicism and Emotion, she addresses the essential concept of "assenting" to our impressions:
"An impression is what one might call a mere thought, a linguistically formulable notion that one entertains without necessarily being committed to it. What converts thought into belief is a further mental event which is termed variously 'assent' (sunkatathesis), 'judgment' (krisis), or 'forming an opinion' (doxazein). Assent is defined in intentional terms: it is that event in which one either accepts an impression as true or rejects it as false. That assent also has a physical description comes across most clearly in those texts which treat the conditioning factors for assent, whether or not it occurs in any given instance. For those conditioning factors are described both in intentional terms, e.g. as the extend to which the person recognizes valid inferences, and in physical terms as a certain level of tension in the mind material. Thus several sources claim that the assent in the person of perfect understanding [the sage] is characterized by 'strength' or 'good tension,' while the less reliable assents of ordinary persons are 'weak.' The ordinary mind is, as it were, a pushover, yielding easily to impressions which the wise person would resist."
It is of particular importance to note that Stoicism holds that (other than a brief reflexive reaction to incoming impressions that is quickly mastered in reasonable beings) we are in control of whether or not we assent to our impressions. The world tries to impress us, to act upon our minds in various ways. The Stoic knows that nothing can act upon his or her mind without his or her assent.