610. Here at any rate the commonly admitted category of stages of inward vision will close. Thus far and no farther the brain's capacity for presenting visual images can be pushed on under the guidance of the conscious will of man. It is now my business to show, on the contrary, that we have here reached a mere intermediate point in the development of internal vision. These imagination-images, valuable as they are, are merely attempts to control supraliminally a form of vision which - as spontaneous memory-images have already shown us - is predominantly subliminal. The memory-images welled up from a just-submerged stratum; we must now consider what other images also well upward from the same hidden source.

To begin with, it is by no means certain that some of Watt's images of steam-engines did not well up from that source, - did not emerge ready-made into the supraliminal mind while it rested in that merely expectant state which forms generally a great part of invention. We have seen in Chapter III (Genius). that there is reason to believe in such a conveyance in the much inferior mental processes of calculating boys, etc, and also in the mental processes of the painter. In short, without pretending to judge of the proportion of voluntary to involuntary imagery in each several creative mind, we must undoubtedly rank the spontaneously emergent visual images of genius as a further stage of internal vision.

And now we have reached, by a triple road, the verge of a most important development of inward vision - namely, that vast range of phenomena which we call hallucination. Each of our last three classes had led up to hallucination in a different way. Dreams actually are hallucinations; but they are usually hallucinations of low intensity; and are only rarely capable of maintaining themselves for a few seconds (as hypnopompic illusions) when the dreamer wakes to the stimuli of the material world. Imagination-images may be carried to a hallucinatory pitch by good visualisers (see 610 A). And the inspirations of genius - Raphael's San Sisto is the classical instance - may present themselves in hallucinatory vividness to the astonished artist.

611. A hallucination, one may say boldly, is in fact a hyperoesthesia; and generally a central hyperesthesia. That is to say, the hallucination is in some cases due indirectly to peripheral stimulation; but often also it is the result of a stimulus to "mind's-eye vision," which sweeps the idea onwards into visual form, regardless of ordinary checks. It is a familiar remark, indeed, that each idea, according as its motor or its sensory elements predominate, is either a nascent movement or a nascent hallucination. We cannot possibly tell beforehand what kind of stimulus, - healthful or harmful, reinforcement of energy or mere breakdown of inhibition, - will carry on this nascency into actual birth. "Mind's-eye vision," like retinal vision, has a habitual limit, in each case presumably determined by natural selection or otherwise as the limit most convenient for the race, considering the resources of the organism. In some individuals, however, these average limits are greatly overpassed, with or without resultant advantage.

Exceptional keenness of ocular vision, useless to most men, may help the astronomer; exceptional power of inward visualisation, to most men a mere curiosity, may singularly help (as in an instance to which I have often alluded) the pourtrayers from memory of flying birds.

Here, then, is a comprehensive and reasonable way of regarding these multifarious hallucinations or sensory automatisms. They are phenomena of central or cerebral hyperæsthesia - phenomena which must neither be feared nor ignored, but rather controlled and interpreted. Nor will that interpretation be an easy matter. The interpretation of the symbols by which the retina represents the external world has been, whether for the race or for the individual, no short or simple process. Yet ocular vision is in my view a simple, easy, privileged case of vision generally; and the symbols which represent our internal percepts of an immaterial world are likely to be far more complex than any impressions from the material world on the retina.

All inward visions are like symbols abridged from a picture-alphabet. In order to understand any one class of hallucinations, we ought to have all classes before us. At the lower limit of the series, indeed, the analysis of the physician should precede that of the psychologist. We already know to some extent, and may hope soon to know more accurately, what sensory disturbance corresponds to what nervous lesion. Yet these violent disturbances of inward perception - the snakes of the drunkard, the scarlet fire of the epileptic, the jeering voices of the paranoiac - these are perhaps of too gross a kind to afford more than a kind of neurological introduction to the subtler points which arise when hallucination is unaccompanied by any observable defect or malady.

It is, indeed, obvious enough that the more idiognomonic the hallucination is, the more isolated from any other disturbance of normality, the greater will be its psychological interest. An apparently spontaneous modification of central percepts - what phenomenon could promise to take us deeper into the mystery of the mind ?