Episodes of depersonalization function to keep disturbing preconscious thoughts from becoming conscious. The symptom is conceptualized as a compromise between the wish to be "only dreaming" and the need to maintain a waking state in contact with perceptual reality. Restriction of attention is identified as the central mechanism by which the compromise is effected. On this basis an explanation for the alteration in the sense of reality is suggested. Material from a case is given to support these ideas. The author speculates that early efforts to deal with traumatic experience at a time when the distinction between sleeping and waking states is not secure may lead to the symptom choice of depersonalization.