§ 17. =Simple Images.=—Common sense draws a sharp distinction between our present perception of an object or event, and our later revival of it in memory; and psychologists have been accustomed, in the same way, to distinguish the simple sensation, the elementary process in perception, from the =simple image=, the elementary process in memory. In fact, however, _it is very doubtful if there is any real psychological difference between sensation and image_. The statement is often made that the image is weaker, fainter, more fleeting than the corresponding sensation. Thus, the great philosopher David Hume (1711-1776) wrote: “All the perceptions of the human mind resolve themselves into two distinct kinds, which I shall call _impressions_ and _ideas_. [Hume’s terminology is different from ours.] The difference between these consists in the degrees of force and liveliness, with which they strike upon the mind.” Hume himself admits that “in sleep, in a fever, in madness, or in any very violent emotions of soul, our ideas may approach to our impressions; as on the other hand it sometimes happens that our impressions are so faint and low, that we cannot distinguish them from our ideas.” It is certain that sensation and image are often confused; and some writers have accordingly proposed to drop the term ‘image’ and to replace it by ‘secondary sensation.’ Let us look at the facts. There is no department of sense in which sensation stops entirely when its stimulus is removed; in all cases, even in that of sound, the sensation is prolonged, for a longer or shorter time, and either after an interval or without interruption, in what is called the =positive after-image=. Blow out a match in the dark, and wave the glowing stem about; you see complete circles or figures of eight; the sensation persists, although the stimulus has passed from one part of the retina to another. In some departments, the positive is followed by a =negative after-image=; we have already mentioned the antagonistic after-images of sight. So the removal of a continued warm stimulus leaves a sensation of coolness; and the swimming in the head that you feel while twirling round is followed, when you come to rest, by a swimming in the opposite direction. Lastly, the name of =memory after-image= has been given to an experience which is most familiar, perhaps, in the taking of dictation; as you write the words last spoken, the speaker’s voice still rings in your ears; the sound hangs for a few seconds, as if arrested, and your pen is guided by the mental echo. Similarly, an attentive glance at an object may set up a sort of photographic image that remains distinct for several seconds.