 Like other Stoics, Epictetus regards the mind not as a separate incorporeal entity but as consisting of pneuma (“breath” or “spirit”), a material substance with certain remarkable powers. Thus he explains the faculty of vision by the activity of pneuma infused by god into the eyes, as well as by the energy and tensility of the air through which things are seen (2.23.3–4). In an especially striking analogy in 3.3.20–22, he compares the mind to a vessel of water and mental impressions to rays of light entering the water. When the water is disturbed, the ray of light appears to move, but in fact does not; and likewise when a person experiences vertigo, it seems as if the skills and virtues are disrupted, but in fact the disruption is only in the pneuma in which they exist.