Let us attempt then to resolve disease into its successive steps and to find the first which concerns man. Correcting that, we shall have corrected, fully and completely, the process which constitutes disease. By striking at the root we may destroy the entire growth. Vital Energy _Irritability_ is the property of being susceptible to excitement or stimulation. Stimulation is the process of increasing the functional activity of any organ. Inhibition is the act of checking, restraining, or holding back the functional activity of any organ. These definitions, taken from Gould, are here introduced as a necessary preface to an attempt to set forth, without unnecessary reference to, or discussion of, any other theory as to the etiology of disease, the Chiropractic explanation of its presence. Chiropractic maintains that all the chemical and physical activities of the human organism are controlled, directly or indirectly, through a third form of energy transmitted through the Nerve System; that while all three forms of energy are interdependent and closely related in their ultimate expression, one of the three is the _primary_ and most essential form, and especially indicative of life. We may call this third form _Vital Energy_. There are several good reasons for believing that this nerve force is the primary form in which energy is expressed in man and for believing that it controls and directs the others in greater degree than it is controlled and directed by them. Of the four forms of tissue of which the body is composed--connective, epithelial, muscular, and nervous--the latter is the one damage to which is followed by the greatest and most permanent consequences. It is a fact that there are several organs whose removal leads to certain death because of their importance in the general economy of the body, but it is also true that section of the nerves leading to these organs just as certainly causes death by the cessation of their functions. There is no organ in the body aside from the nerves themselves which does not immediately cease to act upon withdrawal of its nerve force and at once begin a process of degeneration or atrophy. Pathologic changes in the Nerve System invariably are followed by pathologic changes in the organs controlled by the diseased segment but the converse is not true. Excitation or inhibition of nerve activity produces corresponding and responsive change in the activity of the organs innervated, but excitation of an organ does not necessarily produce similar changes in the Nerve System. That system possesses the power of inhibiting or permitting responsive action, in other words, the power of _choice_. Research in Comparative Anatomy develops the fact that the differences in power of complex action possessed by different organisms are entirely measurable by differences in the structure and complexity of their nerve mechanisms.