Doppelgängers: Hallucinating Oneself

An even stranger and more complex form of hallucinating oneself occurs in “heautoscopy,” an extremely rare form of autoscopy where there is interaction between the person and his double; the interaction is occasionally amiable but more often hostile. Moreover, there may be deep bewilderment as to who is the “original” and who the “double,” for consciousness and sense of self tend to shift from one to the other. One may see the world first with one’s own eyes, then through the double’s eyes, and this can provoke the thought that he—the other—is the real person. The double is not construed as passively mirroring one’s posture and actions, as with autoscopy; the heautoscopic double can do, within limits, whatever it wants to (or it may lie still, doing nothing at all).

– Oliver Sacks, Hallucinations