Sensations

Epicurus believed that the sensations, i.e. the stimuli that entered our awareness through our senses, are basically truthful.

Epicurus explained the pattern towards awareness as a three-step process: first we e.g. see something, perceiving it simply and naturally through our sense of vision; second, the stimulus received by the eyes is judged rationally by the mind (e.g. Did I really see a centaur, or was it simply a man mounted on a horse?); finally, the stimulus is either confirmed or rejected.

Yet Epicurus was well aware that what our senses perceive can occasionally be deceptive: his classic examples of this phenomenon included the impression that an octagonal tower, seen from afar and through the fog, appears cylindrical instead, or that an oar submerged in water seems crooked, although of course it is straight. In such cases, argued Epicurus, it is not our senses that are at fault, but our faulty reasoning, if we do not "adjust" the visual stimulus we received by using our better judgment, experience, and critical abilities. This process outlined by Epicurus may be what is currently termed the path from mere perception to full cognition.

Epicurus, however, strongly rejected the notion that the senses, and thus the sensations we receive through them, are generally, routinely misleading. If we don't trust our senses, he argued, we are left without any criterion at all, by which to judge what is true and what is untrue. Thus Epicureanism is a fundamentally empirical world-view.