In normal physical and mental health, however, it seems rare for the hallucinatory influence of dreams to extend beyond the hypnagogic state, but any impairment of the bodily health generally, and of the brain in particular, may extend this confusion. Thus in a case of heart disease terminating fatally, the patient, though in health he was by no means visionary or impressionable, became liable during sleep in the day-time to dreams of an entirely reasonable character which he had great difficulty in distinguishing from the real facts of life, never feeling sure what had actually happened, and what had been only a dream. In disordered cerebral and nervous conditions the same illusion becomes still more marked. This is notably the case in hysteria. In some forms of insanity, as many alienists have shown, this mistake is sometimes permanent and the dream may become an integral and persistent part of waking life. At this point, however, we leave the normal world of dreams and enter the sphere of pathology. In the normal persistence of the dream illusion into the hypnagogic state with which we are here concerned, the dream usually presents a possible, though, it may be, highly improbable event. The half-waking or hypnagogic intelligence seems to be deceived by this element of life-like possibility. Consequently a fallacy of perception takes place strictly comparable to the fallacious perception which, in the case of an external sensation, we call an illusion. In the ordinary illusion an externally excited sensation of one kind is mistaken for an externally excited sensation of another kind. In this case a centrally excited sensation of one order (dream image) is mistaken for a centrally excited sensation of another order (memory). The phenomenon is, therefore, a mental illusion belonging to the group of false memories, and it may be termed hypnagogic paramnesia. The process seems to have a certain interest, and it may throw light on some rather obscure phenomena. When we are able to recall a vivid dream, usually a fairly probable dream, with no idea as to when it was dreamed, and thus find ourselves in possession of experiences of which we cannot certainly say that they happened in waking life or in dream life, it seems probable that this hypnagogic paramnesia has come into action; the half-waking consciousness dismisses the vivid and life-like dream as an old and familiar experience, shunting it off into temporary forgetfulness, unless some accident again brings it into consciousness with, as it were, a fragment of that wrong label still sticking to it. Such a paramnesic process may thus also help to account for the mighty part which, as so many thinkers from Lucretius onwards have seen, dreams have played in moulding human action and human belief. It is a means whereby waking life and dream life are brought to an apparently common level.