This image-making is, of course, voluntary sometimes; but most images come without volition on our part, and require no effort that we are conscious of. To call up an image voluntarily requires conscious effort; and to keep it in position while we gaze upon it requires effort that is great in proportion to the time during which it is exerted. Psychologists speak of this act of keeping an image in position as one of giving attention, or paying attention. To perform this act requires the exercise of will, unless the act gives pleasure, or the image suggests danger; in each of these cases, of course, the act is almost involuntary. A man who is observant notes consciously the incidents that are passing around him: he seizes on certain of the millions of pictures passing before him, concentrates their images on his retina, and gazes on each one for a while. Similarly, a man who is contemplative, seizes on certain of the vague mental pictures passing through his mind, concentrates his attention on them, and gazes at each one for a while. We call the former an observant man and the other a thoughtful man. Sometimes an observant man learns a great deal from what he sees, in the same way that sometimes a studious man learns a great deal from what he studies; but the learning of course cannot be accomplished without the assistance of the memory. One is often surprised to see how little some observant and studious men have remembered. Many impressions have been received, but few retained. The thoughtful man, of course, cannot in the nature of things receive so many conscious impressions as the merely observant or studious man; for the reason that he continually seizes on one and then another, and holds each for a time, while he fixes his attention on it. Usually, however, the thoughtful man memorizes his observations or his studies for some specific purpose; he moves the various images about in his mind; and arranges them in classes: for otherwise, the various images would form merely an aggregation of apparently unrelated facts. The value of such aggregations is, of course, enormous; they compose what we call data, and include such things as tables of dates, etc.