It is apparent that in none of the classifications of this second kind is any mention made of the primary action or _modus operandi_ of medicines in the cure of disease, as a necessary basis of such distinctions.
III. OPINIONS CONCERNING THE MODE OF OPERATION OF MEDICINES, AND CLASSIFICATIONS FOUNDED ON THIS.
In this third division are included those writers who have attempted to account for the mode in which medicines produce each their peculiar effects after entering into the blood, and some who have classified them according to their ideas on this point. It is with such theories as these that I am more immediately concerned in this Essay. Such writers have dived into a deeper subject than those who have directed attention to the general effects or tendencies of medicines rather than to the means by which such results are attained. Thus it is not to be wondered at that they have sometimes failed. Those have erred most who have allowed their imaginations to lead them astray from facts, or to guide them in matters which are naturally incomprehensible, to which our reason gives us no clue.
Attempts have been made to account for the _modus operandi_ of therapeutic agents generally, in three different ways.
1. Mechanical theories of the action of medicines were greatly in vogue during the seventeenth and eighteenth centuries. There is a tendency in the human mind to explain every thing; and it was only natural for men who knew little of chemistry or of physiology to resort to the science of physics, which they could comprehend, in attempting the explanation of observed phenomena.
John Locke, in his essay concerning the Human Understanding, published in 1689, gave it as his opinion, that the shapes of the minute particles of medicines were sufficient to account for their several operations.
"Did we know," said he, "the mechanical affections of the particles of rhubarb, hemlock, opium, and a man, as a watchmaker does those of a watch, whereby it performs its operations, and of a file, which, by rubbing on them, will alter the figure of any of the wheels, we should be able to tell beforehand that rhubarb will purge, hemlock kill, and opium make a man sleep." This idea did not originate with the great metaphysician. The first rudiments are to be found in the doctrines of the _Methodic Sect_ among the Romans, a medical branch of the Epicurean school. They held that diseases depended either on constriction or relaxation of the tissues, and that medicines operated by mechanically affecting these conditions.