The living organism possesses a susceptibility to the action of certain general stimuli, such as light, heat, electricity, aliment, atmospheric air, etc. The action and reaction of these stimuli and this susceptibility are the conditions of life. So long as they act upon it in a due relative proportion, as regards intensity and quantity, the equilibrium of the functions is preserved and the organism continues in healthy action. The absolute withdrawal of one of the stimuli for any considerable length to time results in death. A disturbance of their due proportion, in respect of intensity or quantity, produces an abnormal performance of function in the organism- a deviation from health-disease. But these stimuli are continually varying in proportion, or, in other words, the relative susceptibility of the organism is continually changing.
Why doses not disease constantly exist? Because the organism is endowed with either a faculty of provisionally supplementing to a limited extent one stimulus by another, or with a kind of elasticity, a power of enduring for a certain time a disturbance of the equilibrium of these stimuli, and of rebounding to a normal performance of functions again so soon as the natural proportion of the stimuli is restored or the deficiency made up. In this respect, the living organism differs from an inorganic machine, which cannot, in the nature of things, possess any power to endure a disturbance of that equilibrium of forces which is the condition of its normal working without a disorganization from which it has no inherent power to recover.
But, is the organism this elasticity has its limits. This “vis medicatrix naturae” is not inexhaustible. If the due proportion of the stimuli remain too long disturbed, the functions of the organism become permanently deranged at least, to such as extent, that no restoration of the balance of the stimuli will cause a return to their normal performance. The functions are and remain deranged-disease has occurred; or, if we choose to call every deviation from a state of equilibrium disease, then we may say that now disease ensues which has no tendency to revert to health without the intervention of some extraneous influence foreign to the organism and different from the general stimuli aforesaid.
Since, then, the general stimuli will not bring back the organism to a healthy action, a new element must be sought for and introduced, the action of which upon the susceptibilities of the organism may cause a restoration to health. This new element will be a special stimulus. Being foreign to the organism and different from the general stimuli, not only must it act upon susceptibilities in the organism which the latter do not awaken, but the formula which shall express its relations to those susceptibilities, and which shall furnish the rule for its employment, can never be discovered by a study of Physiology, for Physiology concerns herself with the relations of the general stimuli aforesaid and the general susceptibilities of the organism.
This formula of the relations of special stimuli and special susceptibilities can be discovered only by the application of induction to a multitude of instances of the action and reaction of such stimuli and susceptibilities, and confirmed by subsequent deductive verifications. this formula will constitute an empirical law, which will be the law or fundamental principle of Therapeutics. For the application of special stimuli to the diseased organism is the domain of the science of Therapeutics, while all that concerns the restoration and maintenance of a proper equilibrium of the general stimuli appertains to the science of Hygiene.