Lastly, in Part Fifth, Mr. Spencer proposes to consider the _Principles of Morality_, bringing to bear the truths furnished by Biology, Psychology, and Sociology, to determine the true theory of right living. He will show that the true moral ideal and limit of progress is the attainment of an equilibrium between constitution and conditions of existence, and trace those principles of private conduct, physical, intellectual, moral, and religious that follow from the conditions to complete individual life. Those rules of human action which all civilized nations have registered as essential laws--the inductions of morality--will be delineated, and also those mutual limitations of men's actions necessitated by their coexistence as units of society, which constitute the foundation of justice. * * * * * It cannot be doubted that the order here indicated, as it corresponds to the method of nature, is the one which Philosophy must pursue in the future. It combines the precision of science with the harmony and unity of universal truth. The time is past when Biology can be considered with no reference to the laws of Physics; Mind with no reference to the science of Life, and Sociology, without having previously mastered the foregoing subjects. The progress of knowledge is now toward more definite, systematic, and comprehensive views, while it is the highest function of intellect to coordinate and bind together its isolated and fragmentary parts. In carrying out his great plan, therefore, Mr. Spencer is but embodying the large philosophical tendencies of the age. If it is urged that his scheme is too vast for any one man to accomplish, it may be replied: 1st. That it is not intended to treat the various subjects exhaustively, but only to state general principles with just sufficient details for their clear illustration. 2d. A considerable portion of the work is already issued, and much more is ready for publication, while the author is still in the prime of life. 3d. It must be remembered that intellects occasionally appear, endowed with that comprehensive grasp and high organizing power which fits them for vast undertakings. The reader will find at the close of the volume Mr. Spencer's Prospectus of his system. That he who has so clearly mapped out his work is the proper one to execute it, we think will be fully apparent to all who peruse the present volume. An impression prevails with many that Mr. Spencer belongs to the positive school of M. Auguste Comte. This is an entire misapprehension; but the position having been assumed by several of his reviewers, he repels the charge in the following letter, which appeared in the _New Englander_ for January, 1864. _To the Editor of the New Englander_: