Perception, again, is an impression produced on the mind, its name being appropriately borrowed from impressions on wax made by a seal; and perception they divide into comprehensible and incomprehensible: Comprehensible, which they call the criterion of facts, and which is produced by a real object, and is, therefore, at the same time conformable to that object; Incomprehensible, which has no relation to any real object, or else, if it has any such relation, does not correspond to it, being but a vague and indistinct representation... But there is a difference between φαντασία and φάντασμα. For φάντασμα is a conception of the intellect, such as takes place in sleep; but φαντασία is an impression, τύπωσις, produced on the mind, that is to say, an alteration, ἀλλοίωσις, as Chrysippus states in the twelfth book of his treatise on the Soul... But φαντασία is understood to be that which is impressed, and formed, and imprinted by a real object, according to a real object, in such a way as it could not be by any other than a real object... — Diogenes Laertius, *Lives*, VII.35–36 (tr. C. D. Yonge, 1853) **Surface Gloss:** The Stoics distinguish between a *phantasma* (an ungrounded mental figment, as in dreams) and a *phantasia* (a real physical alteration of the soul), making the "comprehensible" (*kataleptic*) impression—stamped unmistakably by a real object—the sole criterion of truth. Run a strict two-bin intake filter on your own cognition: is this thought a **Phantasia** (an alteration forced upon my model by direct contact with an external, recalcitrant fact, carrying details I could not have invented) or a **Phantasma** (an internally generated simulation, anxiety loop, or narrative projection)? Treat any impression that lacks the "could-not-come-from-anything-else" signature of hard reality as a dream-shadow until verified.
— In our words · Source: Lives of Eminent Philosophers (Diogenes Laertius)