§41. =Illusions of Recognition and Memory.=—Psychologically, an illusory memory is a memory, just as an illusory perception is a perception. We speak of _illusion_ when our experience fails to square with what, from our knowledge of external circumstances and of other like experiences, we might have expected; the distinction is therefore practical, not scientific. We shall avail ourselves of it, partly for convenience’ sake, and partly because certain cases of illusion offer special problems to the psychologist. Most of us, probably, have an occasional acquaintance with what is called =paramnesia= or wrong recognition: a definite ‘feeling that all this has happened before,’ sometimes connected with a ‘feeling that we know exactly what is coming,’—a ‘feeling’ which persists for a few seconds and carries positive conviction, in spite of the fact and the knowledge that the experience is novel; Dickens gives an instance in _David Copperfield_. Various explanations have been offered of the phenomenon. It occurs most frequently after periods of emotional stress, or in the state of extreme mental fatigue; that is, at a time when the associative tendencies in the brain are abnormally weak; and it seems to depend, essentially, upon a disjunction of mental processes that are normally held together in a single state of attention. Suppose the following case: you are about to cross a crowded street, and you take a hasty glance in both directions to make sure of a safe passage. Now your eye is caught, for a moment, by the contents of a shop window; and you pause, though only for a moment, to survey the window before you actually cross the street. Paramnesia would then appear as the feeling that you had already crossed; the preliminary glance up and down, which ordinarily connects with the crossing in a single attentive experience, is disjoined from the crossing; the look at the window, casual as it was, has been able to disrupt the associative tendencies. As you cross, then, you think ‘Why, I crossed this street just now’; your nervous system has severed two phases of a single experience, both of which are familiar, and the latter of which appears accordingly as a repetition of the earlier. The illusion will evidently be strengthened if, as is only natural, the casual look at the window does not recur to you. This is an imaginary case, simplified for clearness of exposition; and we cannot be at all sure that the explanation which it suggests is correct; for cases of paramnesia cannot be realised at will, and the nervous condition that leads to them is not favourable to scientific observation; but something of the sort must take place.