The trouble is not alone that men occupy a very narrow set of brain cells with their special interest, and make all their energy pass through that set, but among men who are lacking in a certain insulation in the nervous system, this particular set of cells continues to be active, even at times when they think they are resting or diverting themselves. Unconscious cerebration (the occupation of the mind with subjects when we are not deliberately giving our attention to them) is a common phenomenon in human psychology. With the rise of extreme specialization, it has become even more dangerous than before. In the past unconscious cerebration might mean any occupation, with any one of a number of interests. At the present it is likely to mean concentration of thought on a particular subject with which the brain is prone to be occupied more than is good for it, even during the hours of ordinary labor. It seems worth while to discuss at some length, then, the subject of unconscious cerebration, because it constitutes the pathological physiology of many nervous states that we see in modern life. Frequency of Unconscious Cerebration.--The mind, having been set to work over a given thought, continues at it sub-consciously, even while apparently completely occupied with something else. Most people who devote themselves to the intellectual life have experienced phenomena more striking and going much farther in unconscious cerebration than this. Most writers have a common experience: if they arrange their thoughts on a given subject and then turn aside to something else, they find, when they go over the same subject next day, much more material than came the day before. The thoughts for an article will often gradually accumulate by unconscious cerebration after the process has been consciously started. {135} At intervals during the next few days succeeding the determination to write a certain article (at moments when no conscious thought is being given to it), ideas crop up that help to fill out the original scheme of thought, and if these are jotted down, a good deal of intellectual work is accomplished without the necessity for that labor over a desk that most of us scheme to avoid. The more familiar literary work becomes, the more frequent are these experiences, and one occasionally wakes up with a thought that opens up a new vista and adds valuable material to what has already been accumulated. If the subject is a large one, as for a book, then most writers will probably confess that some of their best thoughts have come in this "hit and miss" fashion rather than at the times when they were seriously applying themselves to elaborating their theme.