The intention of this book is to lay stress on ideas and tendencies that have to be understood and appreciated, rather than on facts that have to be learned by heart. Many authors are not mentioned and others receive scanty treatment, because of the necessities of this method of approach. The book aims at dealing with the matter of authors more than with their lives; consequently it contains few dates. All that the reader need require to help him have been included in a short chronological table at the end.