The influence of causes acting through the mind is often concealed from our view, and even when it can be plainly seen it is difficult to estimate its amount with correctness. Effects are often produced through the mind, which are attributed by the patient, and sometimes by the physician, too, to some remedy that has been administered. Take a very common case. A dyspeptic, who has contracted his disorder from mental effort, or from the anxieties of business, applies to his physician. He prescribes some medicine, and at the same time recommends him to take a journey, or go to some watering place. He returns cured, and he perhaps gives the credit for the most part to the medicine, or to the medicinal waters which he has drank with scrupulous regularity, either of which may have had little if anything to do with the cure, and relaxation and diversion of mind may have been the chief or sole causes of his recovery. This is a palpable instance of erroneous inference; but we shall have but a narrow idea of the influence of mind upon disease, if we confine our view to cases of so decided a character. Its influence is constant in all diseases; sometimes plain to be seen, as in the case just mentioned; often entirely concealed from the most careful scrutiny; and sometimes revealing itself slightly, so that the watchful eye of the physician catches mere glimpses of it, like passing shadows gone in a moment. Besides the secret griefs and troubles that often hinder recovery, there are varying states of mind, some of which the patient may be hardly conscious of himself, that modify in a thousand ways the movements of disease, and the action of remedies. For example, the cordial which is administered is often in part or wholly neutralized by mental depression, while it is essentially aided in its effects by the genial and animating influence of hope. The points to which I have alluded the reader will find fully illustrated in the chapter on the mutual influence of mind and body in disease. I will therefore dismiss them for the present, and will merely recur again for a moment to our chemical illustration. If the retort of the chemist, besides being composed of substances which will act upon its contents, should have residing in it some secret and subtile principle, whose existence is known only by its effects, and which acts both upon the retort itself and on whatever it contains, the results of his experiments would be rendered very uncertain. To follow out the analogy—the human body being the physician’s retort, the mind is just such a secret and subtile principle, acting in an unseen way both on the retort and its contents, modifying therefore the effects of remedial agents, so as to embarrass the physician in his investigations, and render his conclusions uncertain. The fifth class of causes of the uncertainty of medical science remains to be noticed, viz.: individual peculiarities or _idiosyncrasies_, as they are termed.