This section is from the book "A Compend Of The Principles Of Homoeopathy", by William Boericke. Also available from Amazon: A Compend of the Principles of Homoeopathy as Taught by Hahnemann.
His further research showed that the obstacle to the cure seemed to lie in a previous scabious eruption, which the patient frequently acknowledged having had, and from which he often dated all his sufferings. He believed that chronic diseases occurred on the suppression artificially, or disappearance from any cause of a scabious, itching, eruption from the skin in otherwise healthy persons. Itch, in Hahnemann's time, was a term which covered many other affections besides the one now known as scabies or itch. This itch dyscrasia he called Psora, meaning thereby the internal itch disease, with or without any present skin symptoms. It is the source of all varieties of skin diseases, abnormal growths, tumors, deformity, mental diseases, etc. In short, it is the parent of all chronic diseases, with the exception of venereal diseases. It is the oldest, most universal and obstinate of all miasmatic diseases. The leprosy of the Israelites, the epidemic St. Anthony's fire of the middle ages, were but forms of this taint. In these forms the whole malignity seemed to be expended on the skin. Greater cleanliness and generally better modes of living have modified its local manifestations, so that, at the present day, it is more in herpetic and eczematous diseases that we meet with it. The readiness with which these are suppressed, the readiness with which ordinary practice dries up discharges of all kinds, the immense development of local treatment, and the increase of all kinds of specialists, whose tendency is to suppress local manifestations of disease, has driven this psora within to more vital regions, and thus has lead to the great increase of chronic maladies that afflict mankind.
The appearance of skin symptoms, or discharge from a mucous surface, shows that nature is making an effort to localize on the outskirts of the body the morbid process, removed as far as possible from the more vital parts of the organism, where it would be much more mischievous. Therefore, forcing it back into the interior by strong, local treatment must necessarily work detrimentally to a radical and permanent cure.
"Every external treatment of a local symptom whose aim is to extinguish it on the surface of the body without curing the internal miasmatic disease - such, for example, as that of destroying a psoric eruption on the skin by means of ointments, healing up a chancre by the use of caustic, destroying the granulations of sycosis by ligature, excision or the application of a hot iron - is not only useless, but injurious. This pernicious method, in such general use at the present day, is the chief source of the innumerable chronic diseases that oppress the human race. This is the most criminal practice physicians can adopt, and it has, notwithstanding, been very generally practiced till the present time, and taught, ex cathedra, as the only one. § 203, Organon.