Derealization syndrome of mental impairment, in which surrounding seems dull, grey, "not alive", "in pictures", deprived of colors, movement. When expressed derealization appear to doubt the reality of the surrounding world. Derealization often combined with depersonalization (see). To derealization also include special state "never seen" and "already seen", in which familiar atmosphere seems strange, strange, or, on the contrary, strangers, first seen the situation seems familiar, already seen before. Derealization occurs in a variety of mental illnesses.