It is proper also to invert these two problems, and to inquire,
First, Having the deviation given, what the symptoms must be.
Secondly, Having the manner of operation of a successful remedy given, what the deviation must be.
I here use the words symptoms, deviation, and remedy, in the most general sense possible, for the sake of brevity.
Now it is very evident, that the doctrine of vibrations, or some other better doctrine, which teaches the law of action of the nervous system, has a close connexion with all these last four problems. For the nerves enter every part, as well as the blood-vessels; and the brain has as great a share in all the natural functions of the parts, and its disorders, in all their disorders, as the heart, and its disorders, can have; and much more than any other part, besides the heart.
Farther, if the doctrine of association be the necessary consequence of the doctrine of vibrations, in any such manner as I have proposed above, Prop. IX. and XI. it must have a most intimate connexion with the theory of nervous distempers, and some with that of others, on account of the just-mentioned dependence of all the parts on the brain. Or, if we separate these doctrines, still, if that of association be true, of which I suppose there is no doubt, it cannot but be of great use for explaining those distempers in which the mind is affected.
And it seems to me, that agreeably to this, the distempers of the head, spasmodic ones, the effects of poisonous bites and stings, which, as Dr. Mead justly observes, are more exerted upon the nerves than on the blood, receive much light from the doctrine of vibrations, and, in return, confirm it; and that all the disorders of the memory, fancy, and mind, do the same in respect of the doctrine of association.
I do not mean to intimate here, that the rational and indirect solution of the general problem, which comprehends the art of physic, is preferable to the empirical and direct one, where this is to be had; but only, since this cannot be had always, that we ought to proceed in an explicit and scientifical manner, rather than in a confused and popular one. For where practice is silent, physicians must and will have recourse to some theory, good or bad. And if they do not acquaint themselves with the real structure and functions of the parts, with the sensible qualities and operations of medicines, and with the most probable method of explaining both the symptoms of distempers, and the operations of medicines, they must fancy something in the place of these, and reason from such false imaginations, or perhaps from the mere agreements, oppositions, and secondary ideas, of words. The history of diseases, and their cures, is the basis of all; after this come anatomical examinations of the body, both in its natural and morbid states; and last of all, pharmacy; these three answering respectively to the general problem, and the two subordinate ones above-mentioned. And if we reason
