The distinction between sense-experience and thought is not a distinction between separate and independent classes of pres entation. In general neither exists without the other, so that every complete presentation involves both as essential elements. Change and development in the mental life is change and develop ment at once on the side of thought and of sense, each condi tioning, and conditioned by, the other.
Images are secondary and derivative, inasmuch as they presuppose the previous occurrence of sensations. They also, at least when they are definite, more or less resemble the sense-impressions from which they are derived. Hence they have been called "copies" of impressions.
Sensations are commonly defined by reference to the manner in which they are produced, as experiences immediately conse quent upon the passage of an afferent impulse from a receptive organ to some region of the cerebral hemispheres. But such a definition is inappropriate to our present line of approach, the first task of which is to describe experience itself without regard to the conditions under which it may arise. A formal definition, however, is difficult to give. A typical sensation can but be pointed out and attention drawn to certain characteristics in respect of which it resembles other members of the class with which we are concerned. The clearest and most undisputed ex amples are afforded by bodily aches and pains. These are sensa tions both in the most popular and the most technical senses of the term.
Such an experience as a twinge of pain we find to be an occur rence which manifests a certain specific quality, a certain in tensity and a more or less clearly defined extensity and duration. It is also localised or felt in a certain position in a larger spatio temporal whole. It is referred to some part of the body (of which we are aware as a larger sensory complex), and in popular usage the term sensation tends to be restricted to what is so located. Psychologists, however, have found it inconvenient so to restrict it, so that presentations of sound and colour, and other occurrences which manifest the fundamental attributes are comprised within the meaning of the term.
Theoretically, the attributes of sensation might be found in perfect duplicate in the mental image. A flash of light—say from the bursting of a rocket in the sky—is seen to wax and wane and to pass through various changes in quality and extension. The visualiser might therefore, on closing his eyes, "see" the whole performance repeated "in his mind," the *image corre sponding in a peculiar way in every attribute to the original im pression. Practically, however, images are in various ways defec
tive. They lack the definition, the vividness, the clearness, rich ness and complexity of the primary impression. But mere imper fections do not provide a suitable basis of definition, however much they may assist effective discrimination in ordinary life.
More fundamental is the relation of the image to subjective activity. An image of a statue can be "immediately" produced and does not require to be modelled by the hands, and upon this the relation—or want of relation—of the image to concomitant impressions very largely depends.
Images have places relatively to other images within the same extensive field ; so have impressions relatively to other impres sions. But images and impressions are not thus related to each other. An individual's mental picture of a horse may be con tained within his mental picture of a stable, but neither fall within the field of visual impressions which he experiences together with them. If they did, there would be an hallucination, and hallu cinations are not images but sensations. It should be added that the place of physical objects in space cannot be identified with the place either of impressions or of images. When a candle flame is seen doubled two impressions are sensibly experienced with an interval between them ; but there is only one candle flame.