With respect to the classification of Dr. Cullen, we may commence our objections by stating, that the very basis upon which it rests is a mere gratuitous assumption, viz. _that certain medicines act on the fluids of the body_. With the exception of a very few substances, it is now generally admitted that medicines produce their effects by acting on the living materials of which our organs are composed, and not by modifying the specific gravity, or chemical composition, of the fluids which they may happen to evacuate. The origin of this latter opinion is to be traced to the exploded notions of the humoral pathologists, and to the exclusive doctrine of the earlier chemists; for as the former recognised a depraved condition of the fluids as the source of every disease, so did the latter imagine that every remedy operated by producing a chemical change upon its composition; and the remedial value of a medicinal substance was estimated by its effects upon inert matter. Thus were experiments made with different substances upon the blood, and other fluids of the body, in order to deduce, from the results, the nature and extent of their powers as agents upon the living frame; for instance, the _spirit_ and _salt of hartshorn_, as they were found to render the blood more fluid, when added to it, _out of the body_, were indiscriminately administered in almost every complaint, with a view to dissolve that “lentor of the fluids” which was regarded as the more general source of disease. For similar reasons, a tribe of medicines were introduced into practice under the title of _Antiseptics_, for the prevention of a process which very probably never takes place in the living body: the powers of these supposed agents were, as usual, inferred from their effects in resisting and preventing the putrefaction of dead matter. Nor is the distinction assumed by Dr. Cullen, between the action of remedies on the _Simple_ and _Living_ solids less hypothetical. Tonics and astringents may certainly exert a beneficial effect upon the animal fibre, but not by any immediate action on its materials, but through the agency of its living principle.—“_Medicamentum non agit in cadaver._” The classification of Dr. Young, although it presents many points of interest and value, is not altogether free from objection; his classes in some instances are perhaps unnecessarily sub-divided, without a sufficient regard to the primary and secondary operations of the substances which they include. The arrangement of Murray has been adopted in the present inquiry, not as being less objectionable, in a general point of view, but as one which from its simplicity, is better calculated, as a frame-work, if I may so express myself, for the display of those particular facts, the knowledge of which I consider essential for the successful administration of medicinal agents, and for the full comprehension of those practical doctrines which it is the exclusive object of this work to inculcate.