Katalepsis is ‘comprehension’. Also it means ‘grasping’. In Stoic philosophy, this ancient Greek word expresses the notion of an apprehension, an understanding, a cognition.
In fact, for Stoics, our mind is full of impressions (<phantasiai>), some of them true and some others false. They are true when they are truly affirmed, false when they are wrongly affirmed. An istance of the latters is believing that an oar – half dipped in water – is broken because it appears so. When Orestes, in his madness, mistakes Electra for a Fury, he has an impression both true and false. Firstly, ‘true’ because he indeed sees someone (Electra). Secondly, ‘false’ because she (Electra) is not a Fury.
Moreover, Stoics believe that the mind intuitively differentiates between real and false impressions. Therefore, we should not give credit to all of our perceptions. In fact, we should trust only the perceptions showing some special feature of the things as they appear to us. And such perceptions are <kataleptic phantasiai> that is ‘comprehensible perceptions’.
<Katalepsis> is ‘comprehension’. And a <kataleptic phantasia> is that which derives from an object which really exists. Therefore it is a copy of that object and can derive from no other object. For instance, we can see a meadow while walking in a countryside. But we can also see the same meadow in a perfect, gorgeous picture. Now, the first perception comes with perfumes and some breeze. While the second offers to us no scents and no wind. It means that the first perception shows some special features. Thus, it is a <kataleptic phantasia>. Also notice that the picture could well be the picture of a picture.