15     The short duration of the action of the artificial morbific forces, which we term medicines, makes it possible that, although they are stronger than the natural diseases, they can yet be much more easily overcome by the vital force than can the weaker natural diseases, which, solely in consequence of the longer, generally lifelong, duration of their action (psora, syphilis, sycosis), can never be vanquished and extinguished by it alone, until the physician affects the vital force in a stronger manner by an agent that produces a disease very similar, but stronger, to wit a homœopathic medicine. The cures of diseases of many years' duration (§ 46), by the occurrence of smallpox and measles (both of which run a course of only a few weeks), are processes of a similar character.