01
The idea
The Stoics called it phantasia — an impression. Something occurs, and what presents itself to your mind is not the raw occurrence but an appearance of it, arriving pre-loaded with an implied claim: this is dangerous, this is an insult, this is unfair, you have been overlooked.
The crucial point is that the impression is not chosen. It shows up. Nobody is responsible for the arrival of an impression any more than for the weather, and the school never asks you to prevent them.
What the school does ask is that you notice the difference between an impression arriving and a fact being established. The impression says "this is a catastrophe" in the same tone of voice it says "that is a chair". Distinguishing between those two is the entire discipline of assent.
02
Why this matters
If you treat impressions as direct reports of reality, you have no place to intervene. Every reaction becomes automatic and every explanation becomes retrospective — you find yourself justifying a position you never consciously took.
It also explains why arguing with yourself rarely works. You are arguing with the conclusion while the premise sails through unexamined. The premise is where the leverage is.
And it accounts for the strange fact that two people in the same room can end up in different worlds. The event was one. The impressions were two.