Man's attempt to understand mental illness, and especially depression, have historically alternated between two general concepts: a belief in some form of evil spirits that have invaded the body; or of an internal black toxic substance, melancholia. Each age and culture can be found to have devised its own appropriate treatment for depression; to remove the "biochemical" cause of the disease process by means of prayer, exorcism or fire, or to do away with the evil spirit. Psychoanalysis has evolved a concept of depression that deals with ideas about introjects, rather than conceiving of them as concrete toxins or demons. Psychoanalytic treatment is a cognitive technique for "exorcising" certain identifications by delineating them and then neutralizing them through understanding. The superficial similarity of both concepts, albeit substituting a "tangible" substance by an ideational one, helps to explain why it has been so difficult to avoid the temptation to reify psychoanalytic concepts. The Greeks' black humour, the demon, and the mental construct of an ambivalent introject, can be understood as different metaphors of a similar universal concept.