Schopenhauer enforced his argument by reference to the black death “capable of destroying millions of victims.” It is certain that the black death was no other than human plague, which made enormous ravages, in the fourteenth century, for instance, destroying nearly a third of the population of Europe. In those days, no one doubted but that it was a visitation of the Divine wrath, and people gathered in churches to make common supplication. Sacrifices were offered and flagellations took place in the hope of averting the terrible malady. Travellers who have been in the capital of Austria must have seen in one of the chief streets (Graben) a large and unlovely monument, erected in the seventeenth century to commemorate the interposition of Providence in staying one of the great epidemics of plague. Now that science has made known the true cause of plague, our ideas as to the causes of the appearance and disappearance of epidemics are very different. Plague is not the manifestation of the anger of God, but is a scourge due to invasion by a minute organism, discovered simultaneously by Kitasato and Yersin in 1894. The natural history of the microbe has been studied, and we know that it may live not only in human bodies but in the bodies of small rodents, such as rats and mice, which live in association with man. These animals are the source of human infection, and it is necessary to destroy them as completely as possible. There is no doubt but that the arrest of the plague in the seventeenth century was due to the fact that rats and mice had themselves been exterminated by the plague. Plague, which formerly was the most terrible of epidemic diseases, has now become a misfortune against which it is simple to guard ourselves. To secure that end, however, we have not to pray or to scourge ourselves, but to take measures to destroy rats and mice. Moreover serums may be employed; and the use of these is not only prophylactic, but if the disease be not too advanced, is actually curative. The danger of which Schopenhauer spoke may be regarded as definitely averted, and this is due to the advance of medical knowledge. In such countries as British India in which plague still causes great losses, we have to blame the ignorance of the population. Instead of following the course prescribed by science, these people still prefer the rules laid down by the Brahmanistic religion. Their idea of cleanliness and purity is a religious idea, and not that of medicine and bacteriology. It is not surprising that plague still exists in India, but none the less no case is a better instance of the progress of knowledge. Hartmann’s idea as to a progressive increase in the number of diseases rests on no exact grounds, and is in opposition to much that we know. As a matter of fact, as knowledge of hygiene advances and becomes spread among the peoples, diseases become less frequent and less fatal.