Every individual may, strictly speaking, be said to be peculiar to some extent, and there is much force in the popular idea of the benefit resulting from a physician’s being acquainted with his patient’s constitution. But besides these common differences, some have very great peculiarities. A few examples will be sufficient. There are some persons in whom the odor of roses will produce asthma. Ipecac has the same effect in some individuals. Some persons are uniformly made sick by eating strawberries even in small amount. Cases are constantly met with by physicians in which some medicines have a peculiar effect. The various effects produced by opium in different individuals furnish many examples. I call to mind a patient, who though a laboring man of considerable power of endurance, is extremely prostrated by vomiting, by whatever agent it is produced. I once gave him an emetic without knowing this peculiarity. He was so much prostrated, that I supposed that the apothecary had made a mistake, and that he had taken an overdose. But a short time after, I witnessed in him the same effect induced by undigested food, and this revealed the idiosyncrasy in his case. When idiosyncrasies are known, they can be calculated upon. But they are not always known. We cannot be aware of them when they respect the action of remedies which the patient had never taken. And in relation to remedies which produce no marked and obvious effect, peculiar susceptibilities may exist without being readily ascertained. If there be an idiosyncrasy in regard to such a medicine as an emetic, or an opiate, it is easily discovered. But if it exist in regard to a remedy that acts silently and slowly, it may not show itself clearly. The only evidence that we have of its existence may be the fact, that the medicine after a while is observed to fail in producing the effects which we ordinarily expect from it in such cases. And it may be very doubtful whether this failure is to be attributed to this cause, or to some other. Let us recur once more to our illustration from chemistry. If the retorts used by the chemist, (which, I have supposed to carry out the analogy, to be composed of materials which would act upon their contents,) were not all made exactly alike, but varied a little always in their composition, and sometimes considerably, and that too without the variation always being appreciable, this fact would obviously still further complicate his experiments, and render them uncertain in their results. So also the peculiarities in the different human systems, which are the physician’s retorts into which he introduces his agents, must have the same effect upon his investigations.