Why do the muscles of the arm and leg tire and need sleep as a restorer, while those of the heart and lungs are independent of sleep? Dr. W. H. Thomson, in his book on "Brain and Personality," finds an answer to this question in the fact that the latter do their work independently of the human consciousness, while the former are stimulated and directed by the will. He points out that fatigue comes in proportion to the intensity of the mental effort expended. A baby, to whom everything is strange, whose consciousness is absolutely zero at birth, however well developed his body, sleeps five sixths of the time because of the mental efforts needed in his simplest bodily acts. Brain work, the most absorbing task of consciousness, is always the most compelling in the matter of sleep. Not the muscles themselves but the attention, the skill, the mental effort required to direct those muscles, Dr. Thomson says, constitute the reason for sleep, a reason which, to those who labor only with their hands, must seem unutterably sad. He says that while muscle work is the commonest and the simplest, so it is also the most poorly paid and the most degrading, and that while brain work is ennobling and the highest type of labor, it is so difficult of attainment and produced only by such grievous toil that most of us shirk it, even while reproaching ourselves at our lack of capacity and purpose. The pathetic burden of unfulfilled possibilities, he says, is the curse of labor, and only in sleep does man have temporary oblivion through which, for a time, he forgets his work and, as it were, uses sleep as an anaesthetic for the pain of labor, to rise therefrom each morning ready to carry his burdens for another day. Lack of sleep, to those whose brains are active, speedily brings nervous disaster, and the consciousness, from being the active superintendent of the body, becomes inert, and the body drifts like a boat without a pilot. Lack of sleep to those whose work is muscular means a numbness in the nerve cells which guide those muscles, so that they disobey the will or act unreasonably and without direction. But too much sleep, like over-indulgence in any anaesthetic, is only shirking that duty and avoiding that effort to which the higher life calls us, and the sluggard who sleeps more than the tired nerves need is allowing himself to sink deeper and deeper into a slough of despond. He forgets his toil in sleep, but it is only by active, conscious effort when awake that his work may be lifted to the higher plane where the brain is active, where work ceases to be mechanical and a burden, and where that greatest reward of personal satisfaction can be obtained. CHAPTER XIV _THEORIES OF DISEASE_ Disease may be defined as an abnormal condition of the human body, and since there is no one condition of the human body which can be satisfactorily described as normal, there is, therefore, no exact definition of disease.