In Hellenistic philosophy the term phantasiai is information based on sense experience. Plato described phantasiai as a blend of perception and doxa. Aristotle placed phantasiai between perception and thought. For Aristotle phantasiai is based on sense perception and includes mental images, dreams, and hallucinations. The Pyrrhonists, Epicureans, and the Stoics use the term to refer to information received through the senses and arising in thoughts. In translations of Pyrrhonist texts the term is usually rendered as "appearances" but in translations of Stoic texts there is no consensus how to translate the term, with "appearance," "impression," "presentation," and "representation" all in use. In Stoicism the phantasiai represent pre-cognitive judgments originating from our previous experiences or our subconscious thinking. All psychological states and activities, such as mental assent, cognition, impulse, and knowledge are all either extensions or responses to phantasiai. The founder of Stoicism, Zeno of Citium, defined a phantasiai as an imprinting in the hêgemonikon. He suggested that the soul is imprinted by the senses much in the same way as a signet ring imprints its shape in soft wax. The Stoics held that some phantasiai receive assent and some do not. Assent occurs when the mind accepts a phantasiai as true. According to the Stoics, doxa is a weak or false belief. The sage avoids doxa by withholding assent when conditions do not permit a clear and certain grasp of the truth of a matter. Some phantasiai experienced in perceptually ideal circumstances, however, are so clear and distinct that they could only come from a real object; these were said to be kataleptikê. The kataleptic phantasiai compels assent by its very clarity and represents the criterion of truth. The Stoic philosopherEpictetus said this about phantasiai:
Impressions, by which man's mind is struck at first sight of anything that reaches his intellect, are not under his will or control, but thrust themselves on the recognition of men by a certain force of their own; but the assents by which these impressions are recognized are voluntary and depend on man's control. Therefore when some fearful sound of thunder or a falling house or sudden news of some danger or other, or something else of this sort happens, even the wise man is bound to be moved for a while and shrink and grow pale, not from anticipation of any evil, but from rapid and unconsidered movements forestalling the action of the rational mind. Presently, however, the wise man does not assent to such impressions, he does not approve or confirm them by his opinion, but rejects and repels them and does not think that there is anything formidable in them; and this they say is the difference between the wise man and the fool, that the fool thinks that the impressions which at first strike him as harsh and cruel are really such, and as they go on approves them with his own assent and confirms them by his opinion as if they were really formidable, while the wise man, after showing emotion in colour and complexion for a brief moment, does not give his assent, but keeps the opinions which he has always held about such impressions, firm and strong, as of things which do not really deserve to be feared at all, but only inspire an empty and fictitious terror.
Pyrrhonists maintain that phantasiai cannot be relied upon to represent reality. Phantasiai just appear to be real. Katalepsis was denied by the Pyrrhonists and the Academic Skeptics.
