_essential_ to the constitution of the disease, an enquiry which embraces the second question proposed for consideration, namely, what are the particular phenomena which are common to all the varieties of fever? Now in singling out this particular series of events from the great mass, we are liable to several sources of error. In the first place, we may stop too soon in our enumeration; in the second place, we may mistake the adventitious for the essential and the essential for the adventitious, and in the third place, we may overlook the real place which some particular event holds in the series, and so may suppose that to be antecedent which was truly sequent, and consequently assign that as a cause which is only an effect. The first thing to be done then is to ascertain the concourse of symptoms, and the second, to determine the order in which they occur: when these two points have been made out, what is essential and what adventitious, as well as what is the cause and what the effect, become at once clear and certain. But the difficulty lies in discerning amidst the infinite diversity and contrariety of symptoms which the different modifications of fever present, when we may safely assure ourselves that we are in possession of all the essential phenomena. Our guide is _invariableness_ of concurrence. If we can ascertain that a certain number of events _invariably_ take place in every form and every degree of fever, these events will give us the particular phenomena which are common to all the varieties of the disease. If we can further ascertain that these events _invariably_ concur in a certain order, we shall have discovered what events bear to each other the relation of cause and effect. And the establishment of this relation of events, this constant connexion with each other, this uniform antecedence and sequence appears to me to be the only theory after which it is consistent with the principles of sound philosophy to search. If I have endeavoured to establish this connexion, and have thus ventured, as I conceive, in a strictly philosophical sense to propose a theory, in doing so, I have carefully restricted myself to the attempt to deduce a legitimate conclusion from facts previously ascertained. It does appear to me that these three points, namely, the common phenomena, the invariableness of their concurrence, and their mutual relation are satisfactorily established. Whether I shall be able to communicate this conviction to the reader I do not know: but I hope he will at least coincide with me in opinion that this mode of investigating the disease affords us the best chance of arriving at satisfactory results.