Humanity will be fortunate if the pessimistic philosophers prove as wrong about their other grievances as they have proved about disease and medicine. To understand the vast progress made by medicine, it is necessary only to compare the complaint of Schopenhauer with the actual state of affairs. When he spoke of epidemics being due to slight changes of the atmosphere, Schopenhauer obviously was repeating the medical opinion current in his times. Experimental science has proved that he was quite wrong. It has been shown conclusively that two of the great affections of which he spoke, cholera and plague, are due not to chemical changes in the air, but to definite microbes, the natural history of which is known as well as that of any other plant. Cholera is produced by the vibrio, discovered by Koch, a minute organism that lives in water and that enters the human alimentary canal with food or drink. We do not yet know a definite cure for cholera, but we do know how to prevent infection. The most simple mode of guarding against infection is to swallow only material that has been boiled, and to prevent contamination of water or of vessels with fæcal matter containing the Koch’s vibrio. Moreover, in individual cases use may be made of anti-cholera serums. In 1831, if these discoveries had been made, philosophy would have taken a different course. Instead of trembling at the epidemic, and flying to Frankfurt, Schopenhauer would have remained quietly at Berlin, and Hegel would not have ceased to develop his idealism in the university of that town.