The word ‘image’ is unfortunately used, as the foregoing paragraphs have shown, both for the simple image and for groups or clusters of images; thus, the recurrent image and the habitual image are always complex. Summing up our results, with this warning in mind, we may say that positive and negative after-images, memory colours, and synæsthetic images are definitely sensory in character; that the simple images which make up memory after-images, recurrent and tied and habitual images, hallucinations and dreams, appear to be of the same kind; and that the simple images which compose our ideas of memory and imagination may or may not be intrinsically different from sensations. The simple image may therefore be defined as _an elementary mental process, akin to sensation and perhaps indistinguishable from it, which persists when the sensory stimulus is withdrawn or appears when the sensory stimulus is absent_. We may say further that, while every normal person has very much the same equipment of sensations, there are great individual differences in the matter of secondary sensations or images; in some cases they are interwoven into the whole tissue of experience, in others they are infrequent or even lacking; we shall see presently how they may be replaced. In general, images of sight and sound are common; then come images of touch and temperature, and then again images of taste and smell, which are uncommon; organic images are very rare. Kinæsthetic images undoubtedly occur, and probably occur frequently; but they are likely to blend with kinæsthetic sensations, and so to escape notice. § 18. =Simple Feelings and Sense-Feelings.=—Many of our experiences are indifferent; but many of them, again, are pleasant or unpleasant. These two words, _pleasant_ and _unpleasant_, denote elementary mental processes of a different sort from sensations and images; they are known as =simple feelings=. The term ‘feeling’ is itself even more ambiguous than the term ‘image’; it is natural to speak of ‘feeling’ a strain or effort, a warmth or cold; but we shall henceforth use it only in its technical meaning, to indicate the way in which stimuli affect us, pleasantly or unpleasantly. We must discard altogether the words _pleasure_ and _pain_, although they have long been current as the names of the simple feelings, and although they are much less clumsy than pleasant and unpleasant. We discard them because pain is a sensation (p. 43); and pains, while usually unpleasant, may at times be pleasant; the scratching that relieves an itch and the nip of the wind on a brisk winter’s day are both pains, but they are also both pleasant.