As many of the diseases which are the subjects of these volumes belong to the class of fevers, the following remarks upon their theory are intended to render the principles and language I have adopted, in the history of their causes, symptoms, and cure, intelligible to the reader. I am aware that this theory will suffer by being published in a detached state from the general view of the proximate cause of disease which I have taught in my lectures upon pathology, as well as from its being deprived of that support which it would receive from being accompanied with an account of the remedies for fever, and the times and manner of exhibiting them, all of which would have served to illustrate and establish the facts and reasonings which are to follow upon this difficult and interesting inquiry. I shall not attempt to give a definition of fever. It appears in so many different forms, that a just view of it can only be given in a minute detail of all its symptoms and states. In order to render the theory, which I am about to deliver, more simple and intelligible, it will be necessary to premise a few general propositions. I. Fevers of all kinds are preceded by general debility. This debility is natural or accidental. The former is the effect of the sanguineous temperament, and exists at all times in many constitutions. The latter is induced, 1. By such preternatural or unusual stimuli, as, after first elevating the excitement of the system above its healthy grade, and thereby wasting a part of its strength, or what Dr. Brown calls excitability, and Darwin sensorial power, afterwards reduces it down to that state which I shall call debility of action. Or,