“The living body,” he observes, “assumes, in many cases, different kinds of diseased action, varying remarkably in different periods of life, without any apparent or known cause; but in the greater number of cases it is generally believed that certain circumstances in the situation or condition of patients, before diseases appear, can be assigned with confidence as their causes. The efficacy of these, however, is seldom established in any other way than simply by the observation that persons known to be exposed to their influence become afflicted with certain diseases in a proportion very much greater than those who are not known to be so exposed. “This kind of evidence is in many _individual_ cases very liable to fallacy, in consequence of the great variety of the circumstances capable of affecting health, in which individuals are placed, and of the difficulty of varying these so as to obtain such observations, in the way of induction or exclusion, as shall be decisive as to the efficacy of each. Hence the importance of the observations intended to illustrate this matter being as extensively multiplied as possible; and hence also the peculiar value, with a view to the investigation of the causes of diseases, of observations made on large and organized bodies of men, as in the experience of military and naval practitioners. All the circumstances of the whole number of men whose diseases are there observed, are in many respects exactly alike; they are accurately known to the observer, and are indeed often to a certain degree at his disposal; they are often suddenly changed, and when changed as to one portion of the individuals under observation, they are often unchanged as to another; and therefore the conditions necessary to obtaining an _experimentum crucis_ as to the efficacy of an alleged cause of disease are more frequently in the power of such an observer than of one who is conversant only with civil life. “But when the necessary precautions as to the multiplication of facts, and the exclusion of circumstances foreign to the result in question, are observed, the efficacy of the remote causes of disease may often be determined _statistically_, and with absolute certainty; and the knowledge thus acquired as leading directly to the _prevention_ of disease, is often of the greatest importance, especially with a view to regulations of medical police. And if the human race be destined, in future ages, to possess greater wisdom and happiness in this state of existence than at present, the value of this knowledge may be expected to increase in the progress of time; because there are many diseases which the experience of ages has brought only partially within the power of medicine, but the causes of which are known, and under certain circumstances may be avoided; and the conditions necessary for avoiding them are in a great measure in the power of _communities_, though at present beyond the power of many of the individuals composing these.