There is no science, in which there has been so much theorizing, as there has been in that of medicine. Its history seems to be almost altogether a history of untenable theories. These theories are at least the prominent objects that present themselves to view. Every period has had its favorite theory, which has exerted its influence upon the general medical mind. Almost every great name in medical history is associated with some celebrated hypothesis. And it would seem that sometimes the attention of the whole profession has been almost exclusively directed to the strife between the advocates of opposing theories. This overweening attachment to theories has been a very great obstacle to the advancement of medicine as a science. It has turned the medical mind away from the legitimate pathway of discovery, and the strict observation of facts has been neglected in the contemplation of mere fancies. It is true of medicine, as it is of every other science, that every advance which has been made has been effected by observation, and by observation alone. It is the good observer, and not the mere ingenious theorizer, who has made these advances. And if the theorizer has added anything to the store of knowledge, it is only when he has come down from his airy flight of fancy to the drudgery of humble common observation. He has for the time forgotten his favorite theory, and has subjected hypothesis to its proper subserviency to observation, in suggesting the points to which that observation may be directed. It is in this way, and in this alone, that many of the authors of theories, escaping occasionally from the domination of a theorizing spirit, have added rich treasures to the storehouse of medical science. Even before the discovery of the circulation of the blood, though medical theories necessarily contained many absurdities, yet many of their advocates were acute and accurate observers; and their facts are valuable, though the theories, which they framed to account for these facts, may appear to us even ridiculous. They collected a great amount of good materials; but instead of erecting with them a structure full of beauty and symmetry, capable of resisting the commotions of ages, they reared a motley pile which easily tumbled into ruins. They were men of the most persevering industry. They seemed to forget entirely that “of making many books there is no end; and much study is a weariness of the flesh.” Quartos and folios were produced in abundance, full of mixtures of wisdom and folly, as incongruous as were some of the old prescriptions with their hundred or more ingredients. And as some of the articles, which composed these hundred-headed enemies of disease, are now among our most valuable remedies, so there are portions of those strange compounds of fact and speculation, which will always stand as monuments of genius and industry. They are among the principles which form the basis of medicine. The student, as he