It has always appeared to me that a proper and sufficient distinction is never, or rather, has never, been made, between the predisposing and the exciting causes of disease--an error which leads to much confusion and to contradictory conclusions on all sides. _Par exemple_, in a Report on the epidemic pestilence which raged some years ago in Ireland, it was unanimously determined, (so says the Report,) “that the predisposing causes were, rapid atmospherical vicissitude, low marsh and other effluvia.” Here we have the exciting cause, ‘atmospheric vicissitude,’ jumbled together with a predisposing cause, ‘marsh effluvia;’ while on the other hand, in recent Reports on cholera from various authorities, we have marsh and other effluvia assigned as the exciting causes, which, on reflection, will strike any one as being obviously fallacious, inasmuch as if marsh and other effluvia are to be considered as exciting causes, instead of predisposing to disease, we should scarcely ever find those places free from pestilence where these matters are supposed to be engendered, whereas we see various localities in South America,--in British Guiana, for instance, and even in Ethiopia,--which have been condemned as the hot-bed and nursery of pestilence, where putrefaction is supposed to concoct and concentrate its most lethal poisons,--still enjoy their seasons of salubrity: it is, I repeat, from the absence of such distinctions as to the exciting and the predisposing causes, coupled with the non-consideration of the great variety of circumstances, both past and present, local and general, individual and common, which are influential in the production of disease, that so much uncertainty and disputation as to the causes of epidemic pestilences have arisen. Such distinctions will appear to be more important and necessary for the prevention and mitigation of epidemic pestilence or disease, when properly considered, inasmuch as the predisposing causes, which are in a great degree under our control, are always in existence or operation to a greater or less extent, while the exciting causes may be said to be of only occasional occurrence. As before observed, the causes of diseases are described as the predisposing and the exciting. The former, the predisposing causes, I understand to be,--the want of light; impure air (especially from defective ventilation), in which are included malaria and all other noxious vapours, from whatever source arising; a scanty and impoverishing diet; and, though last, not least, habit of body, induced by the irregular and artificial life of man in a state of civilization, which, by enervating the system, induces the predisposition to disease by rendering the human frame more susceptible to external impressions; the grand excitants of disease, such as elemental disturbance, consisting of variations of temperature and the state of the surrounding medium, as to hygrometric influence, electrical tension, &c.