As the physical body is made up of physical atoms, so our mind is composed of vibrations; every activity of the mind is thought. Now thoughts are of two kinds. First there is imagination. This is an activity of the mind as well as thought, but in imagination the activity is not controlled by the will. When a person is resting in a chair without thinking about anything in particular, the mind has a habit of wandering. In this respect it is like a wild or untamed horse that 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 wanders about on various lines just as it pleases, yet at the same time following the lines to which the mind is accustomed. This is why a musician's imagination naturally dwells on music and on musical things, and an artist's imagination on artistic things. A thief's imagination will dwell on how to rob and that of a writer on what he has been writing about. All this is imagination; that is, it is not controlled by the will.