It is well known that there are very great differences between the visualising powers of different individuals. Some few seem to be capable of calling up mental pictures of what they have seen, possessing a vividness, distinctness, and wealth of detail, little short of actual vision. But the accounts which these people give of themselves must certainly be accepted in many instances cum grano salis. They are usually untrained in introspection, and they probably do not express themselves with rigorous precision. In any case we must make a point of distinguishing between what a man is capable of in the way of visualising when the occasion requires him to do his best and the imagery which enters into his ordinary trains of thought. We shall see at a later stage that the habitual recall of all the concrete detail of actual perception would in ordinary thinking, such as takes place by means of words, be not only a superfluity, but an encumbrance, destroying efficiency. A man who can call up mental pictures equal in distinctness to the reality is no more likely to do so habitually, than a man who can take very long leaps is likely to substitute these for ordinary walking. Setting aside certain exceptional cases as not yet sufficiently investigated, we may affirm that ordinary visual imagery is more or less sketchy and blurred in comparison with actual vision. In some men, including the best introspective psychologists, such as Fechner, it is so very blurred and sketchy that it could scarcely become more so, without ceasing to exist altogether.* The mental pictures of these persons can scarcely be called pictures at all. They are rather the indescribably attenuated ghosts of pictures. They are, to use Fechner's language, "airy, unsubstantial and vaporous." Persons possessing a much higher visualising power than Fechner will readily recognise the aptness of these terms as applied to the greater part of their own visual imagery.

* There are a few exceptional cases, in which the power of visualising appears to be completely absent. Mr. Welton, of the Yorkshire College, Victoria University, assures me that he does not possess even the rudiments of visualising capacity.

Very poor visualisers often find the greatest difficulty in indicating what it is that they actually see with the mental eye. Thus one of James's pupils, asked to call up a picture of his breakfasttable, replies, — "There is nothing definite about it. Everything is vague. I cannot say what I see. I could not possibly count the chairs, but I happen to know that there are ten. I see nothing in detail. The chief thing is a general impression that I cannot tell exactly what I do see."+ This is a somewhat extreme case. But it brings out the point which most requires to be emphasised in this connexion. The indistinctness of mental imagery is to a large extent of a quite peculiar character. It is different in kind from the indistinctness of percepts such as may be due to dimness of light, distance, and the like. It is also different in kind from the indistinctness of positive and negative afterimages in the various phases through which they pass. An ideal image is sketchy and schematic, because it contains only an extract from the content of senseperception. But it is a surprise to most people, who subject these images to introspective scrutiny, when they discover how the extract is often made. It becomes quite intelligible to them that Alice in Wonderland could see the grin without the cat. This applies not only to complex objects, but also in the experience of some persons to apparently simple sensible qualities such as colours and sounds. I attempt to recall a certain definite shade of red and I succeed. On comparing the imaged red with the perceived, I am able to identify the two as the same colour. But they are the same with a difference which does not wholly consist in absence of sensational intensity. There is a "filling in" in the percept which is nonexistent in the idea. What this "filling in" may be I cannot say. All that I am confident about is that it is conspicuously present in the percept and conspicuously absent in the image.*

+Op. cit., vol. ii., p. 54.

* I am also confident that the "filling in" does not wholly consist in accompanying motor and organic sensations.

The comparative indistinctness of images is traceable to various causes. It is partly due to what Dr. Ward has called "obliviscence." Some parts of the percepts have disappeared from the ideal representation, simply because of a deficiency in our power to retain or at least reproduce them. The vagueness of the mental image is also increased by what Dr. Ward calls "reduplication." It is the product, not of a single perception, but of a plurality of perceptions which agree only in certain points, and differ in others. Only the points of agreement are recalled in a fixed and definite manner. The divergent details by their very divergence obstruct the process of reproduction. Hence, so far as they are concerned, the image is vague and fluctuating. "One who had seen the queen but once would scarcely be likely to think of her without finding the attendant circumstances recur as well; this could not happen after seeing her in a hundred different scenes."*

* Ward, Article "Psychology," Encyclopaedia Britannica, ninth edition, xx., p. 62.

But there is a still more important reason for the comparative indefiniteness of ideal revival. It would be not only useless, but disadvantageous, to recall all the details of actual perception. A train of ideational thought is in its character cognitive. It takes place in the service of some practical or theoretical interest. Only so much need be revived as may be required by the dominant interest of the moment; all else being irrelevant would be a mere encumbrance, hindering and embarrassing the course of mental activity. If I wish to recall what I did yesterday, in order to find out how far I have fallen short of the moral ideal, or for any other practical reason, a few minutes will probably suffice for retrospect. But how is it that I can recall in a few minutes experiences which occupied twelve hours? Only by omission. "We simply make an outline sketch, in which the salient characters of things and events and actions appear, without their individualising details. Mere forgetfulness in part helps to make this possible"; but there is much also which I do not forget, and yet do not recall. I pass it over simply because it would not help me, being irrelevant to my guiding interest. "If I picture myself as eating my breakfast at the beginning of the day, it is enough to have a generic image of the breakfasttable and the succession of particular incidents which took up the halfhour spent in eating. Hence it is possible for me to recall the whole event of taking breakfast, which occupied half an hour, in the fraction of a minute, and then to pass on to something else." * In general, mental imagery is more detailed and vivid in persons whose interests are concrete rather than abstract. The savage, the uneducated person, and the poet or artist, have usually far more power at least of mental visualisation and often of other modes of imagery than the mathematician or the philosopher. As we noted above, persons habituated to abstract thinking have often little or no ideal imagery, except reproductions of words.