What was the elevated note of scientific medicine which gradually superseded the empirical method? While empirical medicine believed in blood-letting and blistering, scientific medicine elevated and illustrated the ancient principle which had been forgotten, and which contained all the new wisdom in a synthesis: the medicinal force of nature, _vis medicatrix naturae_. A natural power of fighting and conquering illness exists in the living organism, and it is to this that we must look in order to construct rational medicine; he who believes that the doctor and the medicine cure the sick is an empiricist; but he who knows that it is "only the organism" that can produce the cure, and that therefore we must protect and assist the force which nature gives for our salvation, is a scientist. Now the sum of treatments necessary to protect the natural forces of defense and reorganization in positive medicine, are much more minute and are diffused in much vaster fields than the old empiricism. The great number of specialists who replace the single type of doctor of the last century, is sufficient to emphasize the enormous difference in practise which the new tendency involves. It is interesting also to give a glance at the progress which has been made in medicine; it has begun to cure diseases; and thence it has gone on to discover the laws of normal physical life, and to show the healthy how to preserve their health. When it reached this point it found that the same measures which are necessary for preserving health are the best for curing disease; because it is the same source of life which gives health and the _vis medicatrix naturae_. Thus, for example, the rational diet of to-day is not only a hygienic measure which all should adopt in order to keep themselves in health, but the most important factor in the cure of illness. Dietetics, whether for the victims of gout, pellagra, fever, tuberculosis, or diabetes, is of primary importance; lithia salts, caffeine, and creosote are useless in comparison. The modern tendency is to reject these poisonous remedies altogether, and to substitute the natural remedies of rest, medical gymnastics, hydropathic treatment, and, above all, climatic treatment. Psychiatry and neuropathology have introduced the treatment of work: that is, a course of orderly intelligent activity, to give occupation to individuals who begin to show signs of mental failure. By degrees, as progress is made in this direction, the conception of "natural healing" will triumph--the ever clearer conception, that is to say, of the forces which sustain life. It is only Nature which can do everything, and if the doctor is to become useful he must follow in her footsteps and serve her with increasing fidelity. It is natural that investigation should lead to attempts at interpreting these forces upon which health depends, and these studies of "immunity" have been the most brilliant, widely diffused and scientific of all medical studies.