It must be obvious to the reader, that those who go through this round of experimenting, before they put themselves under the care of an intelligent physician, not only lose valuable time by so doing, but generally inflict upon themselves positive harm. The remedies which they have used, if they have had no good effect, have helped to _fasten_ the disease upon the system, and have increased its severity. They have done this by irritating the system, and, of course, the diseased organs, and by extending the complaint far beyond its original limits. You have seen that, through the sympathy existing between different organs, disease becomes extended and complicated. Well-directed treatment has a tendency to prevent this extension of disease: mere blind experimenting, on the other hand, is apt to promote it; and if it does not have this effect, the patient is very fortunate. 3. I pass now to the consideration of the third class of causes which render medicine an uncertain science, viz., natural changes, arising from the tendency which exists in the system to throw off disease, appropriately called the _vis medicatrix naturæ_, or curative power of nature; and, in connection with this, the tendency to a definite limit, which is manifest in many diseases, as, for example, small pox, measles, hooping cough, scarlet fever, &c. To recur to our chemical illustration. I have said that it would add vastly to the uncertainty of the results of the chemist’s experiments, if the retort, into which he puts his substances to be experimented upon, could itself act upon these substances, and thus modify their action upon each other. The body of the patient may be considered as the physician’s retort, and the diseases and the remedies introduced, as the materials contained in it. Under this head we are to examine certain principles which reside in this retort, and which have a constant and important influence upon diseases and their remedies, modifying, sometimes manifestly, and sometimes secretly, their action upon each other. I will speak first of the tendency to throw off disease, the _vis medicatrix naturæ_. I need not spend time in proving to you the existence of such a tendency. It requires not the exercise of any scientific acumen to discover it. It is obvious to the most superficial observer. And yet the extent to which it operates is far from being properly appreciated, even by medical men; and much less is it by those who are out of the profession. The changes which it produces are constantly confounded with the effects of remedies; and this is one of the chief sources of the errors which encumber the annals of medical experience.