Thus the question arises: What is the cause of _disease_? The question does not apply to any one particular form of disease or class of diseases, but to disease generally, as a concrete term meaning any disorder which may manifest itself by individual disturbances in the body; for such disturbance is but a variation in quantity or quality of one general disturbance, a variation in the mechanism that controls the work of keeping the existing cells in proper condition and replacing those cells which are constantly being destroyed. It is a variation in the process of _regeneration, which we term life_. METABOLISM is the process which is constantly going on in the human system, whereby the cells that have been consumed by oxidation are removed through the excreta--the faeces, the urine, the perspiration, and the exhalations from the lungs--to be replaced by new ones. _Metabolism_, means change of matter. It signifies the course by which nutritive material, or food, is built up into living matter. This process is accomplished through the blood, which distributes the necessary material to all parts of the body where cells need to be replaced and carries away the consumed portions. In the marvelous performance of its functions, when properly supplied, it carries the elements that are essential to regeneration in the correct proportions. When not properly supplied, these proportions become incorrect and foreign formations may arise which are disturbing to the organism. In nature there is a constant tendency to counterbalance disturbances in the proper proportion and by distribution of cell building material to restore the normal condition. We may thus speak of the overwhelmingly curative tendency of nature. Metabolism is the function of the body which most constantly requires attention. So, therefore, it is always through the blood that we must assist nature in the process of counterbalancing and rectifying or healing abnormal conditions. It follows then, that, despite the apparent variety in _constitutional_ diseases, they are all practically the same. They are all disturbances of metabolism through some irregularity in the quantitative or qualitative condition of the blood. Professor Jacob Moleschott, the great physiologist, has crystallized this truth in the immortal words: "One of the principal questions to be always asked of the physician is this: How may good healthy and active blood be obtained? View the question as we may, we shall be forced to acknowledge openly and explicitly or guardedly and indirectly that our volition, our sensations, our strength, and our pro-creative powers are dependent upon our blood and our blood upon our nutrition." If such unity exists, why then the great difference in the human organs? How is it that a bone in its stonelike hardness is essentially the same as the exquisitely sensitive eye?