derangement in the mechanism of perception, and has nothing to do with what the patient is thinking of. In the second, one may have recourse to the theory of subconscious states and assume that every hallucination had a true mental existence, if not conscious, then subconscious, before being brought to the upper consciousness. The first is the old orthodox theory of illusion, and, to make it intelligible, I must say a little of the normal processes of perception.
Our sense-impressions are primarily initiated, as I have said, by currents from the periphery of the body, but their final complexion is only in part determined by the peripheral currents; it owes much to the condition in which the cortex happens to be and to the manner in which it is constituted. We may roughly compare it to a penny-in-the-slot machine. Without the penny there would be no response, but the precise character of the response is determined by the constitution of the machine. The case of the brain is similar, but infinitely more complex. Most of our sense organs send in very complex currents: from the eye, for instance, we get currents which, taken alone, would cause sensations of color, touch, and movement—the slightest change in the number and relative adjustment of these currents, even though it be so slight that we can not possibly be aware of it as a change in the simpler sensations, will totally change the character of the sense-impression. These sensory currents are like the keys and stops of an organ, and any one who knows just what stops and keys to manipulate can get any response he pleases. Thus the technical part of painting consists in so imitating the ordinary sensory determinants of vision by means of colors on a flat surface as to produce that cortical process which is usually produced by a real thing.
It is not often possible to trace the operation of these factors in our sense-impressions. Each seems an indivisible mental whole. But sometimes we can distinguish them. In the first and third of the three types of hallucination which I have already analyzed, the character of the hallucination is clearly determined by the first or central factor, and its development also seems to be due to some central factor or factors. In the second of the three the character is determined as before by the central factor, while its development to sensory intensity is due to its accidental coincidence with the arrival of a sensory current, which is a peripheral factor. These are all termed hallucinations centrally initiated, or true hallucinations. In the types which I shall now take up the character is always chiefly determined by peripheral currents, and, presumably, they are also responsible for the sensory intensity of the image. These hallucinations are termed peripherally initiated, or simply illusions.