The successive instalments of which this volume consists, were issued to the subscribers at the following dates:—Part I. (pp. 1–80) in October, 1860; Part II. (pp. 81–176) in January, 1861; Part III. (pp. 177–256) in April, 1861; Part IV. (pp. 257–334) in October, 1861; Part V. (pp. 335–416) in March, 1862; and Part VI. (pp. 417–504) in June, 1862. _London, June 5th, 1862_ ----- Footnote 1: One of these generalizations is that currently known as “the Conservation of Force;” a second may be gathered from a published essay on “Progress: its Law and Cause;” a third is indicated in a paper on “Transcendental Physiology;” and there are several others. Footnote 2: The ideas to be developed in the second volume of the _Principles of Biology_ the writer has already briefly expressed in sundry Review-Articles. Part IV. will work out a doctrine suggested in a paper on “The Laws of Organic Form,” published in the _Medico-Chirurgical Review_ for January, 1859. The germ of Part V. is contained in the essay on “Transcendental Physiology:” See _Essays_, pp. 280–90. And in Part VI. will be unfolded certain views crudely expressed in a “Theory of Population,” published in the _Westminster Review_ for April, 1852. Footnote 3: Respecting the several additions to be made to the _Principles of Psychology_, it seems needful only to say that Part V. is the unwritten division named in the preface to that work—a division of which the germ is contained in a note on page 544, and of which the scope has since been more definitely stated in a paper in the _Medico-Chirurgical Review_ for Jan. 1859. Footnote 4: Of this treatise on Sociology a few small fragments may be found in already-published essays. Some of the ideas to be developed in Part II. are indicated in an article on “The Social Organism,” contained in the last number of the _Westminster Review_; those which Part V. will work out, may be gathered from the first half of a paper written some years since on “Manners and Fashion;” of Part VIII. the germs are contained in an article on the “Genesis of Science;” two papers on “The Origin and Function of Music” and “The Philosophy of Style,” contain some ideas to be embodied in Part IX.; and from a criticism of Mr. Bain’s work on “The Emotions and the Will,” in the last number of the _Medico-Chirurgical Review_, the central idea to be developed in Part X. may be inferred. Footnote 5: Part IV. of the _Principles of Morality_ will be co-extensive (though not identical) with the first half of the writer’s _Social Statics_. CONTENTS. PART I.—THE UNKNOWABLE. CHAP. PAGE I.— RELIGION AND SCIENCE 3 II.— ULTIMATE RELIGIOUS IDEAS 25 III.— ULTIMATE SCIENTIFIC IDEAS 47