Precisely similar to this is the error of those who for the most part belong to the same school, and who attribute the essence of fever to a morbid condition of the blood. The blood may be diseased in fever, but if it be so, these writers do not _know_ it, or at least they do not adduce any evidence that they are in possession of such knowledge: they do not appear so much as to have questioned chemistry; at all events, it is certain that they have hitherto received no satisfactory answer. There is no evidence on record that the alleged determination of the blood takes place in every type and every degree of fever: and if there were it would still be but one event among many, and one that occurs late in the series, and therefore could possibly be nothing more than an effect. In like manner those who maintain that inflammation of the brain is the sole cause of fever, assume as an established and admitted fact the universal and invariable existence of inflammation of the brain in this disease. Inflammation of the brain, without doubt, is demonstrable of many individual cases, and of some whole types: but beyond this there is no proof that the generalization can be carried: the evidence indeed in regard to many cases is entirely against the assumption, and is as complete as negative evidence can well be: consequently it must be admitted that even this hypothesis, in the present state of our knowledge, is founded on the error of assigning to the whole genus what belongs only to particular species: and it would be trifling with the reader to attempt to prove, that this is still more certainly and strikingly true with regard to inflammation of the mucous membrane of the stomach and intestines—an affection which in innumerable cases in which its existence is certain, clearly appears on the slightest examination of the succession of events, to be an effect and not a cause. No comprehensive view can be taken of fever, no just conclusion can be arrived at relative to its nature and seat until it be studied with a consciousness of the liability to such errors and a vigilant endeavour to avoid them. The present investigation has been undertaken with a deep consciousness of the danger and a watchful and unremitting care to avoid it. Even if the effort prove to be without success, the example can scarcely remain without use.