Post-hypnotic hallucinations can be _postponed_ at will. That is to say, a constant watchfulness is exercised by the subject, so that if, for example, the hypnotiser tells him that he will (when awakened) poke the fire when the hypnotiser has coughed three times, the awakened subject, although knowing nothing of the order in his waking state, will be on the look-out for the coughs, amid all other disturbances, and will poke the fire at the fore-ordained signal.[82] Moreover, when the post-hypnotic suggestion is executed there will often be a slight momentary relapse into the hypnotic state, and the subject will not afterwards be aware that he _has_ (for instance) poked the fire at all. This means that the suggested act belongs properly to the hypnotic, not to the normal chain of memory; so that its performance involves a brief reappearance of the subliminal self which received the order. Another characteristic of these suggested hallucinations tells in exactly the same direction. It is possible to suggest no mere isolated picture,--a black cat on the table, or the like,--but a whole complex series of responses to circumstances not at the time predictable. This point is well illustrated by what are called "negative hallucinations" or "systematised anæsthesiæ." Suppose, for instance, that I tell a hypnotised subject that when he awakes there will be no one in the room with him but myself. He awakes and remembers nothing of this order, but sees me alone in the room. Other persons present endeavour to attract his attention in various ways. Sometimes he will be quite unconscious of their noises and movements; sometimes he will perceive them, but will explain them away, as due to other causes, in the same irrational manner as one might do in a dream. Or he may perceive them, be unable to explain them, and feel considerable terror until the "negative hallucination" is dissolved by a fresh word of command. It is plain, in fact, throughout, that some element in him is at work all the time in obedience to the suggestion given,--is keeping him by ever fresh modifications of his illusion from discovering its unreality. Nothing could be more characteristic of what I have called a "middle-level centre" of the subliminal self--of some element in his nature which is potent and persistent without being completely intelligent;--a kind of dream-producer, ready at any moment to vary and defend the dream.