§ 5. Relative Independence of Percept and Image. — Gazing at the blue sky, we may, as Dr. Ward observes, mentally picture a portion of it as red instead of blue. Now it is very important to note that most people, while they are imaging the sky as red, do not cease to see it as blue. The red does not get between them and the sky so as to hide its blueness. Similarly, in calling up with closed eyes a visual image, most persons find that this image does not form part of the grey field which is due to the retina's own light. It may sometimes appear to be merging itself in the grey field. But when this happens it is in reality disappearing altogether. The more distinct it is, the more disconnected and independent it appears relatively to the sensations which have their source in the state of the retina.

The case is similar with other senses. I can imagine how the fingers which are now holding my pen would feel if they were dipped in warm water. But the mental image does not annul actual sensation. Similarly, I can clearly distinguish a mentally articulated word, however faint it may be, though my ears are simultaneously assailed by a deafening din. I can also articulate a word mentally when my organs of speech are motionless or engaged in uttering other sounds.

Facts of this kind show that percepts and images possess a relative independence. This can be accounted for if we suppose that the nervous tracts excited in perceptual process are not wholly coincident with those excited in ideational process.

This view is borne out by pathological cases. We have already quoted cases in which the power of recalling ideal images, visual, tactual and auditory, was apparently nonexistent, whereas the corresponding perceptual processes were comparatively intact. Instances of the converse are not wanting. Wilbrand describes the case of a lady who, sitting in her armchair with eyes closed, could distinctly describe streets and houses in their right order, though she could not recognise them when she saw them, and was soon hopelessly lost when left to find her way by herself.*

* See Professor Ward's Article on "Assimilation and Association, ii.," Mind, Oct. 1894.

The question as to the relation of the nervous seats of sensations and percepts on the one hand, and of ideas on the other, is still a vexed one. But the most probable conclusion appears to be that, though they are continuous and more or less overlap, they are by no means necessarily coincident.

In any case it is plain from ordinary experience that the existence of percepts does not imply the possibility of corresponding images. Persons who have little or no power of visual imagery can see actual objects as well as the best visualisers. Similarly, those who have very limited power of mentally reviving sounds may have quite keen auditory perception. Few people, if any, have in a considerable degree the power of calling up mental images of organic sensations. In animals generally, welldeveloped perceptual powers may be combined with little or no capacity for ideal revival.