We will not hesitate to say that this is one of the most important books ever given to man. At age 83, it was no accidental production, but a profound masterpiece produced over fifty years of the most intense reflection and thirty years for teaching on the subject as president of colleges and as professor of mental philosophy while displaying the deepest virtue and usefulness. His biographers wrote: 'The History is not a conpendium of what previous philosophies had maintained but rather a classification of all philosophies into four types, three of which Mahan rebuts and the fourth, that of commonsense realism, he stoutly defends.'