physiology through vivisection, experimental phyisiology, and chemistry, and are apt to believe that before us all the world was in darkness. But if the light of the present day does not dazzle us so much as to prevent our reading the history of medicine aright, we shall be surprised at the uniformity of the pictures of disease bequeathed to us by ancient writers, and of their identity with those that nature furnishes us to-day. It is not the diseases, but the theories invented to explain them, that have undergone the greatest mutations. These and the medicines employed to cure them have varied as continually as the dogmas that were used to explain the one and the other. Thus it came to pass that, as I have already suggested, medical art has always been unstable, while the facts that form the basis of medical science have remained essentially unchanged.
In medicine, as in every other department of knowledge that is founded upon a study and correlation of natural phenomena, this double quality exists—a substratum of positive, demonstrable, and imperishable truth, and a superstructure of more artificial and transient materials imposed by the instinctive longing of the mind for explanations and laws, and, in default of substantial principles, for theories however crude and hypotheses however wild. Theories in the natural sciences are no more essential to them than the clothing we wear is essential to our structure and functions; but we have come so thoroughly to identify men by their garb that we should scarcely recognize them in a nude condition. And so we are prone to confound the substantial facts of pathology and therapeutics with the theories that envelop them and claim to represent them. This blindness to the true nature and aims of science and art is a serious impediment to our acquiring a just conception of either. What has been said of another department of knowledge is equally true of medicine: "The scientific writer should confine himself to describing and picturing what he sees, without attempting to frame any theory, even in the innermost recesses of his mind. Without this precaution the most careful observer is liable to become a dupe of 'expectant attention,' and to see things in accord with his preconceived notions rather than with the facts" (The Nation, January 15, 1896, p. 107). There is a wide difference between facts and formulas. Yet there are many young physicians, as well as callow students, who feel able to tell you all about the work done by every organ of the body; how the chyle is converted into blood, and the blood into living tissue, indeed, into a score of tissues all elaborated from the same identical fluid which
