It would appear from these considerations, that there are certain circumstances, or temporary situations of constitution, which invite infection, and render its effect more certain and violent in one case than another. There are artificial methods, however, of obtruding it, as it were, upon the constitution, though not particularly disposed, or even though averse to receive it; and may not this, in some measure, account for the greater safety of some diseases when communicated by inoculation, than when caught in the natural way? But these, as well as many other facts in animal nature, do not admit of a satisfactory explanation upon any principle as yet known. Even the most common operations of the body, such as digestion and generation, when considered in their causes and modes of action, are so obscure and mysterious, as to be almost beyond the reach of rational conjecture. A little reflection will teach us the utmost modesty with regard to our knowledge of such things; for nature seems to have innumerable ways of working, particularly in the animal functions, to which neither our senses can extend, nor perhaps could our intellects comprehend them. Had we not, for instance, been endowed with the sense of sight, nothing could have led us even to suspect the existence of such a body as light; and there may be numberless other subtile and active principles pervading the universe, relative to which we have no senses, and from the knowledge of whose nature and exigence we must for ever be debarred. We have, indeed, become acquainted with electricity by an operation of reason; and animals have lately been discovered to which the electric fluid serves as a medium of sense through organs calculated to excite it, and to receive and convey its impressions. But there are few subjects we can study that are more subtle and obscure than the influence of one living body on another. There is a familiar instance of the great subtilety of animal effluvia, and also of the fineness of sense in a dog’s being able to trace his master through crowds, and at a great distance; and we can conceive that infectious matter may adhere, and be communicated in a similar manner. We have endeavoured to illustrate the great obscurity of its operation by an allusion to generation, digestion, and other animal functions, with which it is equally obscure and inexplicable. It is similar to generation in this, that its influence does not pass from one species of animal to another; for the poison of the plague, that of the small pox, that of fever, and the venereal disease, do not affect brutes[47], nor do the infectious diseases of brutes affect different species of them, nor the human species. The only exception to this, that we know of, is the bite of a mad dog.