PROLEPSIS is ‘preconception’. Therefore, this ancient Greek word refers to the “preliminary concept” of something. Epikouros (Epicurus) and then the Stoics use the term to indicate basic general notions.
Epikouros suggests three criteria of truth: sensations (<aisthêsis>), preconceptions (<prolepsis>) and feelings or passions (<pathê>). These are the three ways through which Epicureans think we gain ‘knowledge’.
Epicureans believe that sensations never deceive. Therefore, sensations are the first method of truth. When sensory inputs seem to cheat, the stimuli themselves are true and what is wrong is our assessment of the stimuli.
<Prolepsis> is ‘preconception’. And preconceptions are people’s ideas of what things are. As an example, the preconception of a dog is somebody’s idea of what a dog is. A notion like that originates in one’s mind through sensory input over time (meeting dogs for years). When the word indicating the object (in our example ‘dog’) is in use, the preconceptions arise into the subject’s mind. Thanks to preconceptions we are able to evaluate the things we perceive. Preconceptions are also Epicureans’ solution to Platon’s (Plato) paradox in the Meno. Plato says that learning needs us to already have ‘knowledge’ of what we are learning. Otherwise, he sustains, we could not understand when we successfully learn the information. Thus, preconceptions are the sort of ‘pre-knowledge’ that learning requires in Platon’s argument.
Thirdly, feelings or passions (<pathê>) are how we perceive pleasure and pain. Indeed, they are like sensations because they are ways of perception. But what they perceive are our internal states and not the external things we perceive in sensations. Diogenes Laertius says that feelings are how we determine an action. In fact, if we believe that something might give us pleasure, we pursue that thing. Equally, if we believe that something is painful, we avoid that thing.