According to the Stoics, the mind is constantly being bombarded with impressions. Some of these impressions are true and some false. Impressions are true when they are truly affirmed, false if they are wrongly affirmed, such as when one believes an oar dipped in the water to be broken because it appears so. When Orestes, in his madness, mistook Electra for a Fury, he had an impression both true and false: true inasmuch as he saw something, viz., Electra; false, inasmuch as Electra was not a Fury. Believing that the mind instinctively discriminated between real and false impressions, the Stoics said that one ought not to give credit to everything which is perceived, but only to those perceptions which contain some special mark of those things which appeared. Such a perception then was called a kataleptic phantasia, or comprehensible perception. The kataleptic phantasia is that which is impressed by an object which exists, and which is a copy of that object and can be produced by no other object. Cicero relates that Zeno would illustrate katalepsis as follows:
he would display his hand in front of one with the fingers stretched out and say "A visual appearance is like this"; next he closed his fingers a little and said, "An act of assent is like this"; then he pressed his fingers closely together and made a fist, and said that that was comprehension ; but then he used to apply his left hand to his right fist and squeeze it tightly and forcibly, and then say that such was knowledge, which was within the power of nobody save the wise man
Katalepsis was the main bone of contention between the Stoics and the two schools of philosophical skepticism during the Hellenistic period: the Pyrrhonists and the Academic Skeptics of Plato's Academy. These Skeptics, who chose the Stoics as their natural philosophical opposites, eschewed much of what the Stoics believed regarding the human mind and one's methods of understanding greater meanings. To the Skeptics, all perceptions were acataleptic, i.e. bore no conformity to the objects perceived, or, if they did bear any conformity, it could never be known.
