Its presence even in these lower phases of intensity may be detected if we pass from the comparison of sensations with sensations to the comparison of sensations with the sensory element of the image. If we look at a sheet of white paper, and then, closing our eyes, call up a mental picture of the paper, its brightness as actually seen may be revived with approximate accuracy in the image. But if we again open our eyes, and pass from the mental picture to the actual percept, we may note in the moment of transition a difference which can only be described by saying that the image does not strike the mind as the actual percept does. We may vary the experiment by first calling up mentally the image of an electric light, and then looking at a dimmer object, such as a candleflame. The imaged brightness of the electric light is greater than that of the candleflame; but the actual sensation of brightness which we have in looking at the candleflame enters and persists in consciousness in a different manner from the mental picture of the electric light. It strikes the mind with some degree of force and liveliness; whereas the mental image does not strike the mind in the same way.

Our conclusion is that at bottom the distinction between image and percept, as respectively faint and vivid states, is based on a difference of quality. The percept has an aggressiveness which does not belong to the image. It strikes the mind with varying degrees of force or liveliness according to the varying intensity of the stimulus. This degree of force or liveliness is part of what we ordinarily mean by the intensity of a sensation. But this constituent of the intensity of sensations is absent in mental imagery. Since it is distinctive of sensation, we may call it sensational intensity. Sensational intensity may depend on the suddenness of an external impression. A slight sound, when we are totally unprepared for it, may enter consciousness in a violent manner; but in the main sensational intensity increases or decreases concomitantly with the intensity of the stimulus. The smaller it is the less effectively will it serve as a mark distinguishing percept from image. Hence when it is very slight it may be practically inoperative. Thus we may fail to distinguish between a very slight sound as actually heard, and the mental representation of it. The possibility of this has been shown experimentally; but it does not, as has been supposed, constitute a valid argument for regarding the distinction between image and percept as merely one of degree.

(b) Distinctness. Images as compared with percepts have for the most part a sketchy or schematic character. Part of the filling in of the actual sense experience fails to reappear in its reproduction, which is therefore blurred and hazy.

This does not apply without exception to all mental imagery. Most persons, if not all, can reproduce in a precise and delicately differentiated manner certain kinds of experience. The internal language by which trains of thought are habitually carried on in human beings is usually a very precise reinstatement of signs used in the interchange of ideas between different persons. In most cases, probably, words as mentally reproduced are fairly exact counterparts of words as actually spoken. Both sound and motor articulation are revived in a precise and clearcut way. The image lacks sensational intensity; but its qualitative content is indistinguishable from that of the percept. In some persons the motor activity of articulation is very precisely revived, but the auditory element is absent or almost absent. Others mentally envisage printed or written characters either in addition to, or instead of, internal speech.

Even those persons who can reproduce articulate sounds with maximum distinctness may be unable to recall inarticulate noises except in the vaguest manner. When they make the attempt, they tend to substitute some imitation by the human voice for the noises themselves.

The schematic character of ideal representation is best exemplified in the mental reproduction of the appearance of material things as they are presented to sight and touch, and explored by movements of the eye and hand. In most persons this reproduction is predominantly visual, though some depend mainly on motor and tactual revivals. We shall here only consider visual imagery. This includes motor revival: for the "inward eye" follows the contours of objects and scans their parts successively much as the bodily eye does.