One more cause of inaccurate observation remains to be noticed. It is one, however, which, at the present time at least, is so uncommon, that it cannot have any extensive influence. I refer to scepticism. When it does exist, it not only narrows the limits of knowledge, but actually leads to positive error. The sceptic, in his demand for stern, fixed facts, rejects some facts, which are established by evidence that is sufficient to satisfy any mind possessed of candor and common sense; and the rejection of a well-proved fact, being itself an error, must necessarily lead to other errors. The sceptic too, with all his doubting, is always, to some extent, and on some points, a credulous man. As he doubts on some points against clear evidence, so he will assuredly believe on others against evidence just as clear. His beliefs are no more worthy of confidence than his doubts. The sceptic is therefore disqualified by his scepticism for accurate observation. This is especially true in the practice of medicine, because, as you have seen, there is so much uncertainty in it, and the physician is obliged to base so much of his treatment upon probabilities. Some one has said, that the best guesser is the best practitioner. There is some truth in this remark, though it is of course by no means strictly true. It is well for the physician to guess when facts are wanting, but he must be careful not to esteem his guesses to be facts, as is too often done. In medicine, as well as in every other science, but little mental effort is required to frame theories. All the hard work which is done—the work by which all knowledge is accumulated—is the work of _observation_. It therefore needs a higher order of mind to ascertain facts and their relationships, than it does to theorize. “Any man,” says Pott, an eminent English surgeon, “may give an opinion, but it is not every mind that is qualified to collect and arrange important facts.” It is important that the physician should have at the outset good habits of observation. If he does, every day’s experience will add to his store of facts, and at the same time relieve it from some of the chaff of error which has been brought in unawares. _He will be all the time becoming a better practitioner._ But, on the other hand, if he start with a loose habit of observation, experience will be to him a source of error. He will have no _clean store_ of facts, but he will garner in a strange mixture of facts, and suppositions, and errors; and every day’s experience will add to the difficulty of separating the good grain from the mass of refuse, with which it is mingled. _He will be all the time becoming a poorer practitioner._