now reads the older works in medical science, regards but as matters of curiosity the theories of lentor and viscosity, acrimony, &c.; and picks out from the mass of rubbish the pearls and precious stones, which are almost concealed beneath it. The reader cannot fail to have seen, in the course of my remarks, what I deem to be the province of theory, or hypothesis. It is simply suggestive. This is its true vocation. It _establishes_ no fact, and no principle. It should be the mere handmaid of observation, and should be kept in perfect subjection to her control. It never should be a rival, much less should it have supremacy, as it has too often done, in the domains of science. Thus restricted to its proper sphere, theory is of essential service in extending the boundaries of science. It often suggests the line of discovery. It constantly reaches beyond present knowledge. We theorize, that is, we suppose; then by observation we discover. If we find our hypothesis or supposition to be correct, we discover a positive fact. If we find it not to be, we discover a negative fact, and not valueless because negative—some of our negative facts are worth quite as much as positive ones. The abuse of theory consists in the obliteration of the distinction between what is known, and what is merely supposed. So long as this distinction is carefully preserved, no harm is done by theory. No man was ever more thorough, in maintaining this line of separation between the known and the supposed, than was Newton. “_I shall not mingle conjectures with certainties_,” a passage in a letter announcing his great discovery of the compound nature of light, is a maxim which always governed him, and should govern every searcher after truth in every branch of science. This maxim has evidently been little regarded by the great majority of theorizers in medicine. They have exalted mere conjectures into the same rank with facts; and so far as they have done this, just so far have they exerted an influence to retard the progress of medical science. If all the energy and talent, which have been expended upon theories, could have been directed to the observation of facts, what a mass of rubbish, which now encumbers the science of medicine, would have remained uncollected, and what an amount of pure unadulterated facts, which as yet are undiscovered, would have been garnered into the storehouse of knowledge! As observation then is the only source of improvement in medical science, it is important to inquire what influences are adverse to _skill_ in the observer. The influence of an overweening fondness for theory has already been sufficiently pointed out. I will pass therefore to the consideration of some other circumstances which tend to impair skill in observation.