Dr. Fordyce, in treating of the origin of fever, seems inclined to think that it may arise without any predisposing cause; and after having enumerated the various causes already mentioned, and fully considered them, gives it as his opinion, that “there must undoubtedly be other causes than those which give occasion to the disease, but which are at present _totally unknown_.” In like manner Dr. Moore, speaking of the nervous fever, sums up what may be known concerning the cause of it in the following words. “Upon the whole, we know that people of delicate, exhausted and sickly constitutions, and those whose minds are saddened by depressing passions, are greatly predisposed to this disease, the immediate seeds of which, we also know, may be generated in places where human effluvia are collected and confined. And this is the most essential part of our knowledge respecting the cause of this disease; and even this little is disturbed with uncertainty: for we sometimes meet with instances of people of _robust constitutions_, who are seized with the disease in all its malignity, when they are under no _depressing passion_, when the disease is _not epidemic_, to whom we cannot trace it from any place where the human effluvia could be confined in any uncommon degree, or from any person in the disease, of which perhaps there is no other person ill in the neighbourhood for several miles round; and, in short, when we cannot connect it with any of the causes supposed to be the sources of the distemper. On extraordinary occasions of this kind we have nothing for it but to suppose that, notwithstanding the apparent vigour of the patient, his body has been peculiarly predisposed to catch the infection, and that some contagion, not forcible enough to infect any other person, has by some means, unobserved, been conveyed to him; or, if so many suppositions displease, we may suppose at once that there is in some cases a source of this fever which has not been suspected. For, although the numerous observations that have been made give us the strongest reason to think that human effluvia produce this disease, we have no right to infer that it cannot arise also from some other source.”