I will give the name of _imagination-images_ to those conscious recombinations of our store of visual imagery which we compose either for our mere enjoyment, as "waking dreams," or as artifices to help us to the better understanding of facts of nature confusedly discerned. Such, for instance, are imagined geometrical diagrams; and Watt, lying in bed in a dark room and conceiving the steam-engine, illustrates the utmost limit to which voluntary internal visualisation can go.
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. 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.[102] And the _inspirations of genius_--Raphael's San Sisto is the classical instance--may present themselves in hallucinatory vividness to the astonished artist.