§ 4. Characteristic Differences of Percept and Image.—(a) Intensity. Hume is perfectly right in affirming that percepts differ from images "in the force or liveliness with which they strike upon the mind." But the statement is ambiguous.

We must examine with great care the nature of this force and liveliness, which according to Hume and others is distinctive of sensations.

We cannot affirm that a sound or a colour as mentally revived is always louder or brighter than the corresponding sensation. On the contrary, it would seem that variations in the degree of a sensible quality are reproducible in much the same way as variations in hind. I may mentally recall the brightness of an electric light, and I may then actually look at the comparatively dim flame of a candle. On comparing the image with the percept, I may recognise that the electric light as mentally revived has a higher degree of brightness than the candleflame as actually seen. It is true that the power of representing gradations of sensible quality varies in different persons, just as the power of representing the qualities themselves varies. But good visualisers seem to possess it in a very high degree. A person so endowed, in giving an account of his mental picture of the morning breakfasttable, says: "I have more power to recall colour than any other one thing; if, for example, I were to recall a plate decorated with flowers I could reproduce in a drawing the exact tone, etc. The colour of anything that was on the table is perfectly vivid."*

* James, Principles of Psychology.

Are we then to reject the distinction between images and percepts as respectively "faint" and "vivid"? This is a possible course. There are other differences which may be regarded as ordinarily sufficient to prevent confusion between them. But we ought to hesitate before discarding a distinction generally accepted both by psychology and common sense. On the other hand, if we accept it, we must mean by "vividness" something different from those degrees of sensible quality which may be equally present in the sensible quality as actually perceived and as mentally reproduced. What is this vividness? The answer seems to be contained in Hume's words. According to him the distinctive characteristic of percepts as compared with images is the force and liveliness with which they strike the mind. This "striking the mind" is the essential point. At bottom the difference is a difference of quality, not of degree. Images do not strike the mind in the same way as percepts.

To bring out the nature of the difference it will be most convenient to consider first cases in which it is conspicuous. It is most conspicuous where the sensation breaks in upon consciousness in a violent manner, so as to interrupt and disturb the flow of mental activity. A dazzling flash of lightning or the piercing scream of a steamwhistle may serve as illustrations. The shriek of the steamwhistle invades consciousness in a violently disturbing way. The degree of loudness of the sound might perhaps be mentally reproduced with tolerable accuracy by a person possessed of exceptional powers in this direction. But the mode of occurrence in consciousness would be different. The mentally revived sound would not "strike the mind" like the sound as actually heard. No mere image ever does strike the mind in this manner.

In such experiences as that of the steamwhistle the primary sensation is by no means the only factor at work. The whole organism receives a shock giving rise to a mass of organic and motor sensations. In ideal revival these concomitant sensations fail to be recalled except in a very imperfect way. It may be suggested that it is their presence in the actual sense-experience which gives to this experience its aggressive character. Now it seems evident that they cannot give an aggressive character to the experience unless they possess this character themselves, and as a matter of fact they are highly intrusive and obtrusive. But if organic sensation can "strike the mind" in this way, there is no reason why other sensation should not do so too. The ultimate appeal must be to introspection. This shows in the case of the steamwhistle that the sound itself is aggressive in the same way as the organic sensations which accompany it. The organic sensations follow the beginning of the sound after the lapse of about a second, but the sound itself is aggressive from the outset.

The steamwhistle is an extreme case, involving violent shock and disturbance. But there are abundant instances in which sensations strike the mind without overwhelming it in this painful manner. The chimes of a bell heard as we are passing in front of a church break in upon consciousness with notable force and liveliness. But they need not have a disturbing effect, and they need not be accompanied by conspicuous organic sensations. None the less they have an impressiveness or aggressiveness analogous to that of the steamwhistle. The same holds true generally of sensations produced by a stimulus which is stronger than we are accustomed to. But there is a certain normal level of intensity of stimulus at which and below which we do not naturally notice the aggressive character of the sensation, unless it occurs suddenly and finds us unprepared. At these lower intensities the aggressiveness of the sensation does not under ordinary conditions catch our attention: but it would be wrong to conclude that it has therefore ceased to exist. We do not usually take note of what is familiar to us, but only of what is relatively unfamiliar. It is therefore natural that the characteristic of sense-experience, which is expressed by such metaphors as "striking" the mind or "laying hold" on the mind, should only be noticed when it is present in an unusual degree of intensity.