TO THE SECOND EDITION
It is a pleasure to me to find that a new edition of this book is wanted. I am encouraged to hope that with all its defects it has helped to stimulate thought on first principles. And it has been a further pleasure to me to find that my critics have in general taken this work in the spirit in which it was offered, whether they have or have not found themselves in agreement with its matter. And perhaps in some cases sympathy with its endeavour may have led them to regard its shortcomings too leniently. I on my side have tried to profit by every comment, though I have made no attempt to acknowledge each, or to reply to it in detail. But I fear that some criticisms must have escaped my notice, since I have discovered others by mere chance.
For this edition I have thought it best not to make many alterations; but I have added in an Appendix, beside some replies to objections, a further explanation and discussion of certain difficulties.