This is an activity of the mind, as thought is. But in imagination, the activity is not controlled by the will. When a person is resting in a chair without specially thinking over a subject, the mind has a habit of wandering about. In this respect it is like a wild or untamed horse which runs off into the jungle at will. It goes off without knowing whither or why, for it is its habit just to wander about. So the imagination is not specially directed, and goes about on various lines just as it pleases, but following the lines to which the mind has been accustomed.