The science of medicine is so much a mystery to the common observer, that he cannot, as you have already seen, apply his tests to a direct examination of the physician’s knowledge. He is not competent to make the estimate in this way; and if he is not aware of this, he will certainly be deceived. If he wishes effectually to avoid error, he must apply a touchstone which he himself understands, and not one of which he is profoundly ignorant. What is this touchstone? Plainly this. _Let him observe the mental qualities of the physician, as they are exhibited in regard to any subject, with which he is himself familiar in common with the physician_; and he has here a test upon which he may rely with absolute certainty. He discovers in this way the character of the physician’s mind; and it is just to infer that the mental qualities thus laid open to view, stamp their impress upon the practice of his profession, and give to it its character. No change comes over his mind when he passes from other subjects to that of medicine. The same mental powers are there, and he will observe, think reason, and act, just as you have seen him do in regard to common subjects. Take an illustration from surgery. You see a surgeon set a fractured limb. You cannot judge whether he does it skilfully, because you do not understand how it should be done, so as to bring the broken ends accurately together, and keep them so. But if that surgeon, in passing your house, by some accident breaks the thill of his carriage, you can watch him as he splices the thill, and you can judge, for you are competent to do so, whether he exhibits mechanical talent in the operation. If he does, you can safely infer that the same mechanical talent will be brought into exercise in setting a bone, and that he will set it as skilfully as he spliced the thill. Other talents in a medical man can be tested in a similar manner. The truth, of which I have given this single illustration, is so obvious when plainly stated that it hardly needs to be dwelt upon at all; and yet, it is so often disregarded by the community, in the estimates which are made of physicians, that it may be profitable to illustrate it somewhat at large. In doing this we shall accomplish another important purpose—we shall obtain a clear view of those qualities which are most necessary in the particular calling of a physician. Let us then cursorily notice some prominent characteristics, as they are seen in physicians in your daily intercourse with them on common subjects, and apply the criterion which we have under consideration. Look at the mode in which physicians form their opinions.