Definition
Phantasia is the Greek word the Stoics used for an impression: the immediate appearance of something to the mind, whether through the senses or through thought. An impression is the raw presentation of how things seem before any verdict is passed on them.
Crucially, an impression is not yet a belief. It is a candidate for belief — a proposal the mind can either accept, reject, or examine. The Stoic word for accepting it is assent.
Why it matters
How it works
Impressions arrive constantly and often carry an implicit suggestion — an insult arrives with the suggestion this is harmful, a craving with the suggestion you need this. The untrained mind assents to these suggestions automatically.
The Stoic instead treats each impression as something to be tested. Epictetus advised greeting a strong impression with the words wait for me a little, then asking whether it concerns what is up to us and whether its suggested verdict is accurate. By inspecting the impression before assenting, the person decides what to believe rather than being decided by appearances.