To reproduce after a lapse of seventeen years lectures which at the time they were delivered were in such close contact with the most recent work of that day, may well call for a word of justification. Has mathematics not advanced since then, and are the questions here treated of first importance at the present time? I reply by asking: What is important in the development of mathematics? Is it solely the attainment of new results of potential value, or must not an essential part of the best scientific efforts of each new age be devoted to possessing itself of the heritage of the age that has just preceded it? It is a noble example that Professor Klein has set all ages of mathematicians that, beginning his activity at a time when the contributions of the immediate past were so rich and so unrelated, he was able to uncover the essential bonds that connect them and to discern the fields to whose development the new methods were best adapted. His instinct for that which is vital in mathematics is sure, and the light with which his treatment illumines the problems here considered may well serve as a guide for the youth who is approaching the study of the problems of a later day. WILLIAM F. OSGOOD. Cambridge, December 31, 1910. About the Publisher Forgotten Books is a publisher of historical writings, such as: Philosophy, Classics, Science, Religion, History, Folklore and Mythology. Forgotten Books' Classic Reprint Series utilizes the latest technology to regenerate facsimiles of historically important writings. Careful attention has been made to accurately preserve the original format of each page whilst digitally enhancing the aged text. Read books online for free at www.forgottenbooks.org